Add PrometheusTagTransformer

src/OpenTelemetry.Exporter.Jaeger/Implementation/JaegerTagTransformer.cs

@@ -15,6 +15,7 @@
 // </copyright>
 
 using System;
+using System.Text.Json;
 using OpenTelemetry.Internal;
 
 namespace OpenTelemetry.Exporter.Jaeger.Implementation;
@@ -48,5 +49,5 @@ protected override JaegerTag TransformStringTag(string key, string value)
     }
 
     protected override JaegerTag TransformArrayTag(string key, Array array)
-        => this.TransformStringTag(key, System.Text.Json.JsonSerializer.Serialize(array));
+        => this.TransformStringTag(key, JsonSerializer.Serialize(array));
 }

src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusSerializer.cs

@@ -221,14 +221,14 @@ public static int WriteLabelValue(byte[] buffer, int cursor, string value)
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static int WriteLabel(byte[] buffer, int cursor, string labelKey, object labelValue)
+        public static int WriteLabel(byte[] buffer, int cursor, string labelKey, string labelValue)
         {
             cursor = WriteLabelKey(buffer, cursor, labelKey);
             buffer[cursor++] = unchecked((byte)'=');
             buffer[cursor++] = unchecked((byte)'"');
 
             // In Prometheus, a label with an empty label value is considered equivalent to a label that does not exist.
-            cursor = WriteLabelValue(buffer, cursor, labelValue?.ToString() ?? string.Empty);
+            cursor = WriteLabelValue(buffer, cursor, labelValue ?? string.Empty);
             buffer[cursor++] = unchecked((byte)'"');
 
             return cursor;

src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusSerializerExt.cs

@@ -50,15 +50,25 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
 
                     if (tags.Count > 0)
                     {
-                        buffer[cursor++] = unchecked((byte)'{');
-
+                        var tagsWritten = 0;
                         foreach (var tag in tags)
                         {
-                            cursor = WriteLabel(buffer, cursor, tag.Key, tag.Value);
-                            buffer[cursor++] = unchecked((byte)',');
+                            if (PrometheusTagTransformer.Instance.TryTransformTag(tag, out var result))
+                            {
+                                if (++tagsWritten == 1)
+                                {
+                                    buffer[cursor++] = unchecked((byte)'{');
+                                }
+
+                                cursor = WriteLabel(buffer, cursor, tag.Key, result);
+                                buffer[cursor++] = unchecked((byte)',');
+                            }
                         }
 
-                        buffer[cursor - 1] = unchecked((byte)'}'); // Note: We write the '}' over the last written comma, which is extra.
+                        if (tagsWritten > 0)
+                        {
+                            buffer[cursor - 1] = unchecked((byte)'}'); // Note: We write the '}' over the last written comma, which is extra.
+                        }
                     }
 
                     buffer[cursor++] = unchecked((byte)' ');
@@ -113,8 +123,11 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
 
                         foreach (var tag in tags)
                         {
-                            cursor = WriteLabel(buffer, cursor, tag.Key, tag.Value);
-                            buffer[cursor++] = unchecked((byte)',');
+                            if (PrometheusTagTransformer.Instance.TryTransformTag(tag, out var result))
+                            {
+                                cursor = WriteLabel(buffer, cursor, tag.Key, result);
+                                buffer[cursor++] = unchecked((byte)',');
+                            }
                         }
 
                         cursor = WriteAsciiStringNoEscape(buffer, cursor, "le=\"");
@@ -142,16 +155,23 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
                     cursor = WriteMetricName(buffer, cursor, metric.Name, metric.Unit);
                     cursor = WriteAsciiStringNoEscape(buffer, cursor, "_sum");
 
-                    if (tags.Count > 0)
+                    var tagsWritten = 0;
+                    foreach (var tag in tags)
                     {
-                        buffer[cursor++] = unchecked((byte)'{');
-
-                        foreach (var tag in tags)
+                        if (PrometheusTagTransformer.Instance.TryTransformTag(tag, out var result))
                         {
-                            cursor = WriteLabel(buffer, cursor, tag.Key, tag.Value);
+                            if (++tagsWritten == 1)
+                            {
+                                buffer[cursor++] = unchecked((byte)'{');
+                            }
+
+                            cursor = WriteLabel(buffer, cursor, tag.Key, result);
                             buffer[cursor++] = unchecked((byte)',');
                         }
+                    }
 
+                    if (tagsWritten > 0)
+                    {
                         buffer[cursor - 1] = unchecked((byte)'}'); // Note: We write the '}' over the last written comma, which is extra.
                     }
 
@@ -168,16 +188,23 @@ public static int WriteMetric(byte[] buffer, int cursor, Metric metric)
                     cursor = WriteMetricName(buffer, cursor, metric.Name, metric.Unit);
                     cursor = WriteAsciiStringNoEscape(buffer, cursor, "_count");
 
-                    if (tags.Count > 0)
+                    tagsWritten = 0;
+                    foreach (var tag in tags)
                     {
-                        buffer[cursor++] = unchecked((byte)'{');
-
-                        foreach (var tag in tags)
+                        if (PrometheusTagTransformer.Instance.TryTransformTag(tag, out var result))
                         {
-                            cursor = WriteLabel(buffer, cursor, tag.Key, tag.Value);
+                            if (++tagsWritten == 1)
+                            {
+                                buffer[cursor++] = unchecked((byte)'{');
+                            }
+
+                            cursor = WriteLabel(buffer, cursor, tag.Key, result);
                             buffer[cursor++] = unchecked((byte)',');
                         }
+                    }
 
+                    if (tagsWritten > 0)
+                    {
                         buffer[cursor - 1] = unchecked((byte)'}'); // Note: We write the '}' over the last written comma, which is extra.
                     }
 

src/OpenTelemetry.Exporter.Prometheus/Implementation/PrometheusTagTransformer.cs

@@ -0,0 +1,41 @@
+// <copyright file="PrometheusTagTransformer.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Text.Json;
+using OpenTelemetry.Internal;
+
+namespace OpenTelemetry.Exporter.Prometheus;
+
+internal sealed class PrometheusTagTransformer : TagTransformer<string>
+{
+    private PrometheusTagTransformer()
+    {
+    }
+
+    public static PrometheusTagTransformer Instance { get; } = new();
+
+    protected override string TransformIntegralTag(string key, long value) => value.ToString();
+
+    protected override string TransformFloatingPointTag(string key, double value) => value.ToString();
+
+    protected override string TransformBooleanTag(string key, bool value) => value ? "true" : "false";
+
+    protected override string TransformStringTag(string key, string value) => value;
+
+    protected override string TransformArrayTag(string key, Array array)
+        => this.TransformStringTag(key, JsonSerializer.Serialize(array));
+}

src/OpenTelemetry.Exporter.Prometheus/OpenTelemetry.Exporter.Prometheus.csproj

@@ -18,6 +18,10 @@
     <RunApiCompat>false</RunApiCompat>
   </PropertyGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'net462'">
+    <PackageReference Include="System.Text.Json" Version="$(SystemTextJsonPkgVer)" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="$(RepoRoot)\src\OpenTelemetry\OpenTelemetry.csproj" />
   </ItemGroup>

src/OpenTelemetry.Exporter.Zipkin/Implementation/ZipkinTagTransformer.cs

@@ -15,6 +15,7 @@
 // </copyright>
 
 using System;
+using System.Text.Json;
 using OpenTelemetry.Internal;
 
 namespace OpenTelemetry.Exporter.Zipkin.Implementation;
@@ -36,5 +37,5 @@ private ZipkinTagTransformer()
     protected override string TransformStringTag(string key, string value) => value;
 
     protected override string TransformArrayTag(string key, Array array)
-        => this.TransformStringTag(key, System.Text.Json.JsonSerializer.Serialize(array));
+        => this.TransformStringTag(key, JsonSerializer.Serialize(array));
 }