Fix: Make OtlpExporterOptions pick up env variables

When environment variables are set after IConfiguration is built (during WebHost building), even though they are set before exporters are configured, OtlpExporterOptions does not pick them up.

This patch fixes the issue.

Author: lyra95

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/OtlpSpecConfigDefinitions.cs

@@ -11,6 +11,8 @@ namespace OpenTelemetry.Exporter;
 /// </remarks>
 internal static class OtlpSpecConfigDefinitions
 {
+    public const string EnvVarPrefix = "OTEL_";
+    
     public const string DefaultEndpointEnvVarName = "OTEL_EXPORTER_OTLP_ENDPOINT";
     public const string DefaultHeadersEnvVarName = "OTEL_EXPORTER_OTLP_HEADERS";
     public const string DefaultTimeoutEnvVarName = "OTEL_EXPORTER_OTLP_TIMEOUT";

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptions.cs

@@ -67,8 +67,12 @@ internal OtlpExporterOptions(
         BatchExportActivityProcessorOptions defaultBatchOptions)
     {
         Debug.Assert(defaultBatchOptions != null, "defaultBatchOptions was null");
+        var finalConfiguration = new ConfigurationBuilder()
+            .AddEnvironmentVariables(OtlpSpecConfigDefinitions.EnvVarPrefix)
+            .AddConfiguration(configuration)
+            .Build();
 
-        this.ApplyConfiguration(configuration, configurationType);
+        this.ApplyConfiguration(finalConfiguration, configurationType);
 
         this.DefaultHttpClientFactory = () =>
         {