Support both URL-encoded and non-URL encoded values for OTEL_EXPORTER_OTLP_HEADERS (#5316)

Author: pyohannes

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/CHANGELOG.md

@@ -26,6 +26,10 @@
   typically the case when using the experimental Logs Bridge API.
   ([#5300](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5300))
 
+* URL encoded values in `OTEL_EXPORTER_OTLP_HEADERS` are now correctly decoded
+  as it is mandated by the specification.
+  ([#5316](https://github.com/open-telemetry/opentelemetry-dotnet/pull/5268))
+
 ## 1.7.0
 
 Released 2023-Dec-08

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptionsExtensions.cs

@@ -58,6 +58,9 @@ public static THeaders GetHeaders<THeaders>(this OtlpExporterOptions options, Ac
         var headers = new THeaders();
         if (!string.IsNullOrEmpty(optionHeaders))
         {
+            // According to the specification, URL-encoded headers must be supported.
+            optionHeaders = Uri.UnescapeDataString(optionHeaders);
+
             Array.ForEach(
                 optionHeaders.Split(','),
                 (pair) =>

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

@@ -16,6 +16,8 @@ public class OtlpExporterOptionsExtensionsTests : Http2UnencryptedSupportTests
     [InlineData("key==value", new string[] { "key" }, new string[] { "=value" })]
     [InlineData("access-token=abc=/123,timeout=1234", new string[] { "access-token", "timeout" }, new string[] { "abc=/123", "1234" })]
     [InlineData("key1=value1;key2=value2", new string[] { "key1" }, new string[] { "value1;key2=value2" })] // semicolon is not treated as a delimiter (https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specifying-headers-via-environment-variables)
+    [InlineData("Authorization=Basic%20AAA", new string[] { "authorization" }, new string[] { "Basic AAA" })]
+    [InlineData("Authorization=Basic AAA", new string[] { "authorization" }, new string[] { "Basic AAA" })]
     public void GetMetadataFromHeadersWorksCorrectFormat(string headers, string[] keys, string[] values)
     {
         var options = new OtlpExporterOptions