Change StatusCode to 200 when no metrics are collected

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Bug fix for Prometheus Exporter reporting StatusCode 204
+instead of 200, when no metrics are collected
+([#3643](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3643))
+
 ## 1.4.0-alpha.2
 
 Released 2022-Aug-18

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/PrometheusExporterMiddleware.cs

@@ -80,7 +80,7 @@ public async Task InvokeAsync(HttpContext httpContext)
                     else
                     {
                         // It's not expected to have no metrics to collect, but it's not necessarily a failure, either.
-                        response.StatusCode = 204;
+                        response.StatusCode = 200;
                         PrometheusExporterEventSource.Log.NoMetrics();
                     }
                 }

src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Bug fix for Prometheus Exporter reporting StatusCode 204
+  instead of 200, when no metrics are collected
+  ([#3643](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3643))
+
 ## 1.4.0-alpha.2
 
 Released 2022-Aug-18

src/OpenTelemetry.Exporter.Prometheus.HttpListener/PrometheusHttpListener.cs

@@ -166,7 +166,7 @@ private async Task ProcessRequestAsync(HttpListenerContext context)
                     else
                     {
                         // It's not expected to have no metrics to collect, but it's not necessarily a failure, either.
-                        context.Response.StatusCode = 204;
+                        context.Response.StatusCode = 200;
                         PrometheusExporterEventSource.Log.NoMetrics();
                     }
                 }

test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs

@@ -281,7 +281,7 @@ private static async Task RunPrometheusExporterMiddlewareIntegrationTest(
             }
             else
             {
-                Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+                Assert.Equal(HttpStatusCode.OK, response.StatusCode);
             }
 
             validateResponse?.Invoke(response);

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs

@@ -140,7 +140,7 @@ private async Task RunPrometheusExporterHttpServerIntegrationTest(bool skipMetri
             }
             else
             {
-                Assert.Equal(HttpStatusCode.NoContent, response.StatusCode);
+                Assert.Equal(HttpStatusCode.OK, response.StatusCode);
             }
         }
     }