Output EOF following the OpenMetrics exposition recommendation

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

@@ -11,6 +11,9 @@ instead of 200, when no metrics are collected
   ([#3648](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3648))
 * Added support for OpenMetrics UNIT metadata
   ([#3651](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3651))
+* Added `"# EOF\n"` ending following the [OpenMetrics
+  specification](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md)
+  ([#3654](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3654))
 
 ## 1.4.0-alpha.2
 

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

@@ -11,6 +11,9 @@
   ([#3648](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3648))
 * Added support for OpenMetrics UNIT metadata
   ([#3651](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3651))
+* Added `"# EOF\n"` ending following the [OpenMetrics
+  specification](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md)
+  ([#3654](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3654))
 
 ## 1.4.0-alpha.2
 

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs

@@ -191,27 +191,37 @@ private ExportResult OnCollect(Batch<Metric> metrics)
                         }
                         catch (IndexOutOfRangeException)
                         {
-                            var bufferSize = this.buffer.Length * 2;
-
-                            // there are two cases we might run into the following condition:
-                            // 1. we have many metrics to be exported - in this case we probably want
-                            //    to put some upper limit and allow the user to configure it.
-                            // 2. we got an IndexOutOfRangeException which was triggered by some other
-                            //    code instead of the buffer[cursor++] - in this case we should give up
-                            //    at certain point rather than allocating like crazy.
-                            if (bufferSize > 100 * 1024 * 1024)
+                            if (!this.IncreaseBufferSize())
                             {
+                                // there are two cases we might run into the following condition:
+                                // 1. we have many metrics to be exported - in this case we probably want
+                                //    to put some upper limit and allow the user to configure it.
+                                // 2. we got an IndexOutOfRangeException which was triggered by some other
+                                //    code instead of the buffer[cursor++] - in this case we should give up
+                                //    at certain point rather than allocating like crazy.
                                 throw;
                             }
+                        }
+                    }
+                }
 
-                            var newBuffer = new byte[bufferSize];
-                            this.buffer.CopyTo(newBuffer, 0);
-                            this.buffer = newBuffer;
+                while (true)
+                {
+                    try
+                    {
+                        cursor = PrometheusSerializer.WriteEof(this.buffer, cursor);
+                        break;
+                    }
+                    catch (IndexOutOfRangeException)
+                    {
+                        if (!this.IncreaseBufferSize())
+                        {
+                            throw;
                         }
                     }
                 }
 
-                this.previousDataView = new ArraySegment<byte>(this.buffer, 0, Math.Max(cursor - 1, 0));
+                this.previousDataView = new ArraySegment<byte>(this.buffer, 0, cursor);
                 return ExportResult.Success;
             }
             catch (Exception)
@@ -221,6 +231,22 @@ private ExportResult OnCollect(Batch<Metric> metrics)
             }
         }
 
+        private bool IncreaseBufferSize()
+        {
+            var newBufferSize = this.buffer.Length * 2;
+
+            if (newBufferSize > 100 * 1024 * 1024)
+            {
+                return false;
+            }
+
+            var newBuffer = new byte[newBufferSize];
+            this.buffer.CopyTo(newBuffer, 0);
+            this.buffer = newBuffer;
+
+            return true;
+        }
+
         public readonly struct CollectionResponse
         {
             public CollectionResponse(ArraySegment<byte> view, DateTime generatedAtUtc, bool fromCache)

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs

@@ -270,6 +270,15 @@ public static int WriteMetricName(byte[] buffer, int cursor, string metricName,
             return cursor;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int WriteEof(byte[] buffer, int cursor)
+        {
+            cursor = WriteAsciiStringNoEscape(buffer, cursor, "# EOF");
+            buffer[cursor++] = ASCII_LINEFEED;
+
+            return cursor;
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int WriteHelpMetadata(byte[] buffer, int cursor, string metricName, string metricUnit, string metricDescription)
         {

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

@@ -21,6 +21,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.Http;
+using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
@@ -261,21 +262,18 @@ private static async Task RunPrometheusExporterMiddlewareIntegrationTest(
 
                 string content = await response.Content.ReadAsStringAsync().ConfigureAwait(false);
 
-                string[] lines = content.Split('\n');
+                var matches = Regex.Matches(
+                    content,
+                    ("^"
+                        + "# TYPE counter_double counter\n"
+                        + "counter_double{key1='value1',key2='value2'} 101.17 (\\d+)\n"
+                        + "\n"
+                        + "# EOF\n"
+                        + "$").Replace('\'', '"'));
 
-                Assert.Equal(
-                    $"# TYPE counter_double counter",
-                    lines[0]);
+                Assert.Single(matches);
 
-                Assert.Contains(
-                    $"counter_double{{key1=\"value1\",key2=\"value2\"}} 101.17",
-                    lines[1]);
-
-                var index = content.LastIndexOf(' ');
-
-                Assert.Equal('\n', content[^1]);
-
-                var timestamp = long.Parse(content.Substring(index, content.Length - index - 1));
+                var timestamp = long.Parse(matches[0].Groups[1].Value);
 
                 Assert.True(beginTimestamp <= timestamp && timestamp <= endTimestamp);
             }

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

@@ -135,7 +135,7 @@ private async Task RunPrometheusExporterHttpServerIntegrationTest(bool skipMetri
                 Assert.Equal("text/plain; charset=utf-8; version=0.0.4", response.Content.Headers.ContentType.ToString());
 
                 Assert.Matches(
-                    "^# TYPE counter_double counter\ncounter_double{key1='value1',key2='value2'} 101.17 \\d+\n$".Replace('\'', '"'),
+                    "^# TYPE counter_double counter\ncounter_double{key1='value1',key2='value2'} 101.17 \\d+\n\n# EOF\n$".Replace('\'', '"'),
                     await response.Content.ReadAsStringAsync().ConfigureAwait(false));
             }
             else