[sdk] Generate a consistent instanceId for the lifetime of a process (#4988)

src/OpenTelemetry/CHANGELOG.md

@@ -6,6 +6,11 @@
   `8.0.0-rc.2.23479.6`.
   ([#4959](https://github.com/open-telemetry/opentelemetry-dotnet/pull/4959))
 
+* The `AddService` `ResourceBuilder` extension method will now generate the same
+  `service.instance.id` for the lifetime of a process when
+  `autoGenerateServiceInstanceId` is `true`.
+  ([#4988](https://github.com/open-telemetry/opentelemetry-dotnet/pull/4988))
+
 ## 1.7.0-alpha.1
 
 Released 2023-Oct-16

src/OpenTelemetry/Resources/ResourceBuilderExtensions.cs

@@ -25,6 +25,8 @@ namespace OpenTelemetry.Resources;
 /// </summary>
 public static class ResourceBuilderExtensions
 {
+    private static readonly string InstanceId = Guid.NewGuid().ToString();
+
     private static Resource TelemetryResource { get; } = new Resource(new Dictionary<string, object>
     {
         [ResourceSemanticConventions.AttributeTelemetrySdkName] = "opentelemetry",
@@ -71,7 +73,7 @@ public static class ResourceBuilderExtensions
 
         if (serviceInstanceId == null && autoGenerateServiceInstanceId)
         {
-            serviceInstanceId = Guid.NewGuid().ToString();
+            serviceInstanceId = InstanceId;
         }
 
         if (serviceInstanceId != null)

test/OpenTelemetry.Tests/Resources/ResourceBuilderTests.cs

@@ -68,6 +68,24 @@ public class ResourceBuilderTests
         Assert.Contains(new KeyValuePair<string, object>(ResourceSemanticConventions.AttributeServiceName, "my-service"), resource.Attributes);
     }
 
+    [Fact]
+    public void ServiceResourceGeneratesConsistentInstanceId()
+    {
+        var firstResource = ResourceBuilder.CreateEmpty().AddService("my-service").Build();
+
+        var firstInstanceIdAttribute = firstResource.Attributes.FirstOrDefault(kvp => kvp.Key == ResourceSemanticConventions.AttributeServiceInstance);
+
+        Assert.NotNull(firstInstanceIdAttribute.Value);
+
+        var secondResource = ResourceBuilder.CreateEmpty().AddService("other-service").Build();
+
+        var secondInstanceIdAttribute = secondResource.Attributes.FirstOrDefault(kvp => kvp.Key == ResourceSemanticConventions.AttributeServiceInstance);
+
+        Assert.NotNull(secondInstanceIdAttribute.Value);
+
+        Assert.Equal(firstInstanceIdAttribute.Value, secondInstanceIdAttribute.Value);
+    }
+
     [Fact]
     public void ClearTest()
     {