Self-diagnostics - support formatted message for better human readability (#6411)

Co-authored-by: Rajkumar Rangaraj <rajrang@microsoft.com>

src/OpenTelemetry/CHANGELOG.md

@@ -6,6 +6,21 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Added `FormatMessage` configuration option to self-diagnostics feature. When
+  set to `true` (default is false), log messages will be formatted by replacing
+  placeholders with actual parameter values for improved readability.
+
+  Example `OTEL_DIAGNOSTICS.json`:
+
+  ```json
+  {
+      "LogDirectory": ".",
+      "FileSize": 32768,
+      "LogLevel": "Warning",
+      "FormatMessage": true
+  }
+  ```
+
 ## 1.12.0
 
 Released 2025-Apr-29

src/OpenTelemetry/Internal/SelfDiagnosticsConfigParser.cs

@@ -28,6 +28,9 @@ internal sealed class SelfDiagnosticsConfigParser
     private static readonly Regex LogLevelRegex = new(
         @"""LogLevel""\s*:\s*""(?<LogLevel>.*?)""", RegexOptions.IgnoreCase | RegexOptions.Compiled);
 
+    private static readonly Regex FormatMessageRegex = new(
+        @"""FormatMessage""\s*:\s*(?:""(?<FormatMessage>.*?)""|(?<FormatMessage>true|false))", RegexOptions.IgnoreCase | RegexOptions.Compiled);
+
     // This class is called in SelfDiagnosticsConfigRefresher.UpdateMemoryMappedFileFromConfiguration
     // in both main thread and the worker thread.
     // In theory the variable won't be access at the same time because worker thread first Task.Delay for a few seconds.
@@ -36,11 +39,13 @@ internal sealed class SelfDiagnosticsConfigParser
     public bool TryGetConfiguration(
         [NotNullWhen(true)] out string? logDirectory,
         out int fileSizeInKB,
-        out EventLevel logLevel)
+        out EventLevel logLevel,
+        out bool formatMessage)
     {
         logDirectory = null;
         fileSizeInKB = 0;
         logLevel = EventLevel.LogAlways;
+        formatMessage = false;
         try
         {
             var configFilePath = ConfigFileName;
@@ -107,6 +112,9 @@ internal sealed class SelfDiagnosticsConfigParser
                 return false;
             }
 
+            // FormatMessage is optional, defaults to false
+            _ = TryParseFormatMessage(configJson, out formatMessage);
+
             return Enum.TryParse(logLevelString, out logLevel);
         }
         catch (Exception)
@@ -141,4 +149,17 @@ internal sealed class SelfDiagnosticsConfigParser
         logLevel = logLevelResult.Groups["LogLevel"].Value;
         return logLevelResult.Success && !string.IsNullOrWhiteSpace(logLevel);
     }
+
+    internal static bool TryParseFormatMessage(string configJson, out bool formatMessage)
+    {
+        formatMessage = false;
+        var formatMessageResult = FormatMessageRegex.Match(configJson);
+        if (formatMessageResult.Success)
+        {
+            var formatMessageValue = formatMessageResult.Groups["FormatMessage"].Value;
+            return bool.TryParse(formatMessageValue, out formatMessage);
+        }
+
+        return true;
+    }
 }

src/OpenTelemetry/Internal/SelfDiagnosticsConfigRefresher.cs

@@ -43,6 +43,7 @@ internal class SelfDiagnosticsConfigRefresher : IDisposable
     private int logFileSize;  // Log file size in bytes
     private long logFilePosition;  // The logger will write into the byte at this position
     private EventLevel logEventLevel = (EventLevel)(-1);
+    private bool formatMessage;
 
     public SelfDiagnosticsConfigRefresher()
     {
@@ -138,7 +139,7 @@ internal class SelfDiagnosticsConfigRefresher : IDisposable
 
     private void UpdateMemoryMappedFileFromConfiguration()
     {
-        if (this.configParser.TryGetConfiguration(out string? newLogDirectory, out int fileSizeInKB, out EventLevel newEventLevel))
+        if (this.configParser.TryGetConfiguration(out string? newLogDirectory, out int fileSizeInKB, out EventLevel newEventLevel, out bool formatMessage))
         {
             int newFileSize = fileSizeInKB * 1024;
             if (!newLogDirectory.Equals(this.logDirectory, StringComparison.Ordinal) || this.logFileSize != newFileSize)
@@ -147,16 +148,18 @@ internal class SelfDiagnosticsConfigRefresher : IDisposable
                 this.OpenLogFile(newLogDirectory, newFileSize);
             }
 
-            if (!newEventLevel.Equals(this.logEventLevel))
+            if (!newEventLevel.Equals(this.logEventLevel) || this.formatMessage != formatMessage)
             {
                 if (this.eventListener != null)
                 {
                     this.eventListener.Dispose();
                 }
 
-                this.eventListener = new SelfDiagnosticsEventListener(newEventLevel, this);
+                this.eventListener = new SelfDiagnosticsEventListener(newEventLevel, this, formatMessage);
                 this.logEventLevel = newEventLevel;
             }
+
+            this.formatMessage = formatMessage;
         }
         else
         {

src/OpenTelemetry/Internal/SelfDiagnosticsEventListener.cs

@@ -19,17 +19,19 @@ internal sealed class SelfDiagnosticsEventListener : EventListener
     private readonly Lock lockObj = new();
     private readonly EventLevel logLevel;
     private readonly SelfDiagnosticsConfigRefresher configRefresher;
+    private readonly bool formatMessage;
     private readonly ThreadLocal<byte[]?> writeBuffer = new(() => null);
     private readonly List<EventSource>? eventSourcesBeforeConstructor = [];
 
     private bool disposedValue;
 
-    public SelfDiagnosticsEventListener(EventLevel logLevel, SelfDiagnosticsConfigRefresher configRefresher)
+    public SelfDiagnosticsEventListener(EventLevel logLevel, SelfDiagnosticsConfigRefresher configRefresher, bool formatMessage = false)
     {
         Guard.ThrowIfNull(configRefresher);
 
         this.logLevel = logLevel;
         this.configRefresher = configRefresher;
+        this.formatMessage = formatMessage;
 
         List<EventSource> eventSources;
         lock (this.lockObj)
@@ -229,6 +231,15 @@ internal sealed class SelfDiagnosticsEventListener : EventListener
 
             var pos = DateTimeGetBytes(DateTime.UtcNow, buffer, 0);
             buffer[pos++] = (byte)':';
+
+            if (this.formatMessage && eventMessage != null && payload != null && payload.Count > 0)
+            {
+                // Use string.Format to format the message with parameters
+                string messageToWrite = string.Format(System.Globalization.CultureInfo.InvariantCulture, eventMessage, payload.ToArray());
+                pos = EncodeInBuffer(messageToWrite, false, buffer, pos);
+            }
+            else
+            {
                 pos = EncodeInBuffer(eventMessage, false, buffer, pos);
                 if (payload != null)
                 {
@@ -246,6 +257,7 @@ internal sealed class SelfDiagnosticsEventListener : EventListener
                         }
                     }
                 }
+            }
 
             buffer[pos++] = (byte)'\n';
             int byteCount = pos - 0;

src/OpenTelemetry/README.md

@@ -82,7 +82,8 @@ the following content:
 {
     "LogDirectory": ".",
     "FileSize": 32768,
-    "LogLevel": "Warning"
+    "LogLevel": "Warning",
+    "FormatMessage": "true"
 }
 ```
 
@@ -117,6 +118,25 @@ You can also find the exact directory by calling these methods from your code.
    higher severity levels. For example, `Warning` includes the `Error` and
    `Critical` levels.
 
+4. `FormatMessage` is a boolean value that controls whether log messages should
+   be formatted by replacing placeholders (`{0}`, `{1}`, etc.) with their actual
+   parameter values. When set to `false` (default), messages are logged with
+   unformatted placeholders followed by raw parameter values. When set to
+   `true`, placeholders are replaced with formatted parameter values for
+   improved readability.
+
+   **Example with `FormatMessage: false` (default):**
+
+   ```txt
+   2025-07-24T01:45:04.1020880Z:Measurements from Instrument '{0}', Meter '{1}' will be ignored. Reason: '{2}'. Suggested action: '{3}'{dotnet.gc.collections}{System.Runtime}{Instrument belongs to a Meter not subscribed by the provider.}{Use AddMeter to add the Meter to the provider.}
+   ```
+
+   **Example with `FormatMessage: true`:**
+
+   ```txt
+   2025-07-24T01:44:44.7059260Z:Measurements from Instrument 'dotnet.gc.collections', Meter 'System.Runtime' will be ignored. Reason: 'Instrument belongs to a Meter not subscribed by the provider.'. Suggested action: 'Use AddMeter to add the Meter to the provider.'
+   ```
+
 #### Remarks
 
 A `FileSize`-KiB log file named as `ExecutableName.ProcessId.log` (e.g.

test/OpenTelemetry.Tests/Internal/SelfDiagnosticsConfigParserTests.cs

@@ -72,4 +72,77 @@ public class SelfDiagnosticsConfigParserTests
         Assert.True(SelfDiagnosticsConfigParser.TryParseLogLevel(configJson, out string? logLevelString));
         Assert.Equal("Error", logLevelString);
     }
+
+    [Fact]
+    public void SelfDiagnosticsConfigParser_TryParseFormatMessage_Success()
+    {
+        string configJson = """
+            {
+                "LogDirectory": "Diagnostics",
+                "FileSize": 1024,
+                "LogLevel": "Error",
+                "FormatMessage": "true"
+            }
+            """;
+        Assert.True(SelfDiagnosticsConfigParser.TryParseFormatMessage(configJson, out bool formatMessage));
+        Assert.True(formatMessage);
+    }
+
+    [Fact]
+    public void SelfDiagnosticsConfigParser_TryParseFormatMessage_CaseInsensitive()
+    {
+        string configJson = """
+            {
+                "LogDirectory": "Diagnostics",
+                "fileSize": 1024,
+                "formatMessage": "FALSE"
+            }
+            """;
+        Assert.True(SelfDiagnosticsConfigParser.TryParseFormatMessage(configJson, out bool formatMessage));
+        Assert.False(formatMessage);
+    }
+
+    [Fact]
+    public void SelfDiagnosticsConfigParser_TryParseFormatMessage_MissingField()
+    {
+        string configJson = """
+            {
+                "LogDirectory": "Diagnostics",
+                "FileSize": 1024,
+                "LogLevel": "Error"
+            }
+            """;
+        Assert.True(SelfDiagnosticsConfigParser.TryParseFormatMessage(configJson, out bool formatMessage));
+        Assert.False(formatMessage); // Should default to false
+    }
+
+    [Fact]
+    public void SelfDiagnosticsConfigParser_TryParseFormatMessage_InvalidValue()
+    {
+        string configJson = """
+            {
+                "LogDirectory": "Diagnostics",
+                "FileSize": 1024,
+                "LogLevel": "Error",
+                "FormatMessage": "invalid"
+            }
+            """;
+        Assert.False(SelfDiagnosticsConfigParser.TryParseFormatMessage(configJson, out bool formatMessage));
+        Assert.False(formatMessage); // Should default to false
+    }
+
+    [Fact]
+    public void SelfDiagnosticsConfigParser_TryParseFormatMessage_UnquotedBoolean()
+    {
+        string configJson = """
+            {
+                "LogDirectory": "Diagnostics",
+                "FileSize": 1024,
+                "LogLevel": "Error",
+                "FormatMessage": true
+            }
+            """;
+        Assert.True(SelfDiagnosticsConfigParser.TryParseFormatMessage(configJson, out bool formatMessage));
+        Assert.True(formatMessage);
+    }
 }