Add User-Agent customization support to OTLP Exporter via headers

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptionsExtensions.cs

@@ -27,6 +27,8 @@ public static THeaders GetHeaders<THeaders>(this OtlpExporterOptions options, Ac
     {
         var optionHeaders = options.Headers;
         var headers = new THeaders();
+        string? customUserAgent = null;
+
         if (!string.IsNullOrEmpty(optionHeaders))
         {
             // According to the specification, URL-encoded headers must be supported.
@@ -56,13 +58,33 @@ public static THeaders GetHeaders<THeaders>(this OtlpExporterOptions options, Ac
 
                 var key = pair.Slice(0, equalIndex).Trim().ToString();
                 var value = pair.Slice(equalIndex + 1).Trim().ToString();
-                addHeader(headers, key, value);
+
+                // Extract custom User-Agent to prepend to default
+                if (string.Equals(key, "User-Agent", StringComparison.OrdinalIgnoreCase))
+                {
+                    customUserAgent = value;
+                }
+                else
+                {
+                    addHeader(headers, key, value);
+                }
             }
         }
 
         foreach (var header in OtlpExporterOptions.StandardHeaders)
         {
-            addHeader(headers, header.Key, header.Value);
+            if (string.Equals(header.Key, "User-Agent", StringComparison.OrdinalIgnoreCase))
+            {
+                // Create User-Agent with custom prefix if provided
+                var userAgentValue = string.IsNullOrWhiteSpace(customUserAgent)
+                    ? header.Value
+                    : $"{customUserAgent!.Trim()} {header.Value}";
+                addHeader(headers, header.Key, userAgentValue);
+            }
+            else
+            {
+                addHeader(headers, header.Key, header.Value);
+            }
         }
 
         return headers;

test/OpenTelemetry.Exporter.OpenTelemetryProtocol.Tests/OtlpExporterOptionsExtensionsTests.cs

@@ -4,6 +4,7 @@
 #if NETFRAMEWORK
 using System.Net.Http;
 #endif
+using System.Linq;
 using Microsoft.Extensions.Configuration;
 using OpenTelemetry.Exporter.OpenTelemetryProtocol.Implementation;
 using OpenTelemetry.Exporter.OpenTelemetryProtocol.Implementation.ExportClient;
@@ -133,6 +134,92 @@
         AssertTransmissionHandler(transmissionHandler, exportClientType, expectedTimeoutMilliseconds, retryStrategy);
     }
 
+    [Fact]
+    public void GetHeaders_NoUserAgentInHeaders_ReturnsDefaultUserAgent()
+    {
+        var options = new OtlpExporterOptions
+        {
+            Headers = "Authorization=Bearer token123",
+        };
+
+        var headers = options.GetHeaders<Dictionary<string, string>>((d, k, v) => d.Add(k, v));
+        var userAgentHeader = headers.FirstOrDefault(h => h.Key == "User-Agent");
+
+        Assert.NotEqual(default(KeyValuePair<string, string>), userAgentHeader);
+        Assert.StartsWith("OTel-OTLP-Exporter-Dotnet/", userAgentHeader.Value, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetHeaders_UserAgentInHeaders_PrependsToDefault()
+    {
+        var options = new OtlpExporterOptions
+        {
+            Headers = "User-Agent=MyDistribution/1.0.0",
+        };
+
+        var headers = options.GetHeaders<Dictionary<string, string>>((d, k, v) => d.Add(k, v));
+        var userAgentHeader = headers.FirstOrDefault(h => h.Key == "User-Agent");
+
+        Assert.NotEqual(default(KeyValuePair<string, string>), userAgentHeader);
+        Assert.StartsWith("MyDistribution/1.0.0 OTel-OTLP-Exporter-Dotnet/", userAgentHeader.Value, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetHeaders_UserAgentWithWhitespace_TrimmedAndPrepended()
+    {
+        var options = new OtlpExporterOptions
+        {
+            Headers = "User-Agent=  MyService/2.1.0  ",
+        };
+
+        var headers = options.GetHeaders<Dictionary<string, string>>((d, k, v) => d.Add(k, v));
+        var userAgentHeader = headers.FirstOrDefault(h => h.Key == "User-Agent");
+
+        Assert.NotEqual(default(KeyValuePair<string, string>), userAgentHeader);
+        Assert.StartsWith("MyService/2.1.0 OTel-OTLP-Exporter-Dotnet/", userAgentHeader.Value, StringComparison.Ordinal);
+        Assert.DoesNotContain("  ", userAgentHeader.Value, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void GetHeaders_UserAgentWithOtherHeaders_PrependsCorrectly()
+    {
+        var options = new OtlpExporterOptions
+        {
+            Headers = "Authorization=Bearer token,User-Agent=CustomAgent/3.0.0,Content-Type=application/json",
+        };
+
+        var headers = options.GetHeaders<Dictionary<string, string>>((d, k, v) => d.Add(k, v));
+
+        // Should have Authorization, Content-Type, and User-Agent (from standard headers with custom prepended)
+        Assert.Equal(3, headers.Count);
+        Assert.Contains(headers, h => h.Key == "Authorization" && h.Value == "Bearer token");
+        Assert.Contains(headers, h => h.Key == "Content-Type" && h.Value == "application/json");
+
+        var userAgentHeader = headers.FirstOrDefault(h => h.Key == "User-Agent");
+        Assert.NotEqual(default(KeyValuePair<string, string>), userAgentHeader);
+        Assert.StartsWith("CustomAgent/3.0.0 OTel-OTLP-Exporter-Dotnet/", userAgentHeader.Value, StringComparison.Ordinal);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void GetHeaders_EmptyOrWhitespaceUserAgent_UsesDefault(string userAgentValue)
+    {
+        var options = new OtlpExporterOptions
+        {
+            Headers = $"User-Agent={userAgentValue}",
+        };
+
+        var headers = options.GetHeaders<Dictionary<string, string>>((d, k, v) => d.Add(k, v));
+        var userAgentHeader = headers.FirstOrDefault(h => h.Key == "User-Agent");
+
+        Assert.NotEqual(default(KeyValuePair<string, string>), userAgentHeader);
+        Assert.StartsWith("OTel-OTLP-Exporter-Dotnet/", userAgentHeader.Value, StringComparison.Ordinal);
+
+        // Should not have extra spaces or the empty custom prefix
+        Assert.DoesNotContain("  ", userAgentHeader.Value, StringComparison.Ordinal);
+    }
+
     private static void AssertTransmissionHandler(OtlpExporterTransmissionHandler transmissionHandler, Type exportClientType, int expectedTimeoutMilliseconds, string? retryStrategy)
     {
         if (retryStrategy == "in_memory")