Add Prometheus exporter integration test (#918)

* Add Prometheus exporter integration test

* Refactor

test/IntegrationTests/GraphQLTests.cs

@@ -84,7 +84,7 @@ public class GraphQLTests : TestHelper
 
         int aspNetCorePort = TcpPortProvider.GetOpenPort();
         using (var agent = new MockZipkinCollector(Output))
-        using (Process process = StartTestApplication(agent.Port, arguments: null, packageVersion: string.Empty, aspNetCorePort: aspNetCorePort))
+        using (var process = StartTestApplication(agent.Port, aspNetCorePort: aspNetCorePort))
         {
             if (process.HasExited)
             {

test/IntegrationTests/Helpers/TestHelper.cs

@@ -121,17 +121,22 @@ public abstract class TestHelper
     /// StartTestApplication starts the test application
     // and returns the Process instance for further interaction.
     /// </summary>
-    public Process StartTestApplication(int traceAgentPort, string arguments, string packageVersion, int aspNetCorePort, string framework = "", bool enableStartupHook = true)
+    public Process StartTestApplication(int traceAgentPort = 0, string arguments = null, string packageVersion = "", int aspNetCorePort = 0, string framework = "", bool enableStartupHook = true)
     {
         var testSettings = new TestSettings
         {
-            TracesSettings = new TracesSettings { Port = traceAgentPort },
             Arguments = arguments,
             PackageVersion = packageVersion,
             AspNetCorePort = aspNetCorePort,
             Framework = framework,
             EnableStartupHook = enableStartupHook
         };
+
+        if (traceAgentPort != 0)
+        {
+            testSettings.TracesSettings = new() { Port = traceAgentPort };
+        }
+
         return StartTestApplication(testSettings);
     }
 

test/IntegrationTests/SmokeTests.cs

@@ -18,8 +18,12 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
 using FluentAssertions;
 using FluentAssertions.Execution;
+using FluentAssertions.Extensions;
 using IntegrationTests.Helpers;
 using IntegrationTests.Helpers.Mocks;
 using IntegrationTests.Helpers.Models;
@@ -145,6 +149,39 @@ public class SmokeTests : TestHelper
         }
     }
 
+    [Fact]
+    [Trait("Category", "EndToEnd")]
+    public async Task PrometheusExporter()
+    {
+        SetEnvironmentVariable("LONG_RUNNING", "true");
+        SetEnvironmentVariable("OTEL_METRICS_EXPORTER", "prometheus");
+        SetEnvironmentVariable("OTEL_DOTNET_AUTO_METRICS_ADDITIONAL_SOURCES", "MyCompany.MyProduct.MyLibrary");
+        string defaultPrometheusMetricsEndpoint = "http://localhost:9464/metrics";
+
+        using var process = StartTestApplication();
+
+        try
+        {
+            var assert = async () =>
+            {
+                var response = await new HttpClient().GetAsync(defaultPrometheusMetricsEndpoint);
+                response.StatusCode.Should().Be(HttpStatusCode.OK);
+
+                var content = await response.Content.ReadAsStringAsync();
+                Output.WriteLine("Raw metrics from Prometheus:");
+                Output.WriteLine(content);
+                content.Should().Contain("TYPE MyFruitCounter counter");
+            };
+            await assert.Should().NotThrowAfterAsync(
+                waitTime: 30.Seconds(),
+                pollInterval: 1.Seconds());
+        }
+        finally
+        {
+            process.Kill();
+        }
+    }
+
     private static void AssertNoSpansReceived(IImmutableList<IMockSpan> spans)
     {
         Assert.True(spans.Count() == 0, $"Expecting no spans, received {spans.Count()}");

test/test-applications/integrations/TestApplication.Smoke/Program.cs

@@ -14,10 +14,12 @@
 // limitations under the License.
 // </copyright>
 
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.Metrics;
 using System.Net.Http;
+using System.Threading;
 
 namespace TestApplication.Smoke
 {
@@ -29,6 +31,12 @@ namespace TestApplication.Smoke
         {
             EmitTraces();
             EmitMetrics();
+
+            var longRunning = Environment.GetEnvironmentVariable("LONG_RUNNING");
+            while (longRunning == "true")
+            {
+                Thread.Yield();
+            }
         }
 
         private static void EmitTraces()