Allow additional context to be added when WithHeaders is used in OTLP…

… gRPC traces exporter (#5915)

Fix #5904

CHANGELOG.md

@@ -26,6 +26,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc` now keeps the metadata already present in the context when `WithHeaders` is used. (#5892)
 - `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc` now keeps the metadata already present in the context when `WithHeaders` is used. (#5911)
+- `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc` now keeps the metadata already present in the context when `WithHeaders` is used. (#5915)
 
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->

exporters/otlp/otlptrace/otlptracegrpc/client.go

@@ -229,7 +229,12 @@ func (c *client) exportContext(parent context.Context) (context.Context, context
 	}
 
 	if c.metadata.Len() > 0 {
-		ctx = metadata.NewOutgoingContext(ctx, c.metadata)
+		md := c.metadata
+		if outMD, ok := metadata.FromOutgoingContext(ctx); ok {
+			md = metadata.Join(md, outMD)
+		}
+
+		ctx = metadata.NewOutgoingContext(ctx, md)
 	}
 
 	// Unify the client stopCtx with the parent.

exporters/otlp/otlptrace/otlptracegrpc/client_test.go

@@ -19,6 +19,7 @@ import (
 	"google.golang.org/grpc/backoff"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/encoding/gzip"
+	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/status"
 
 	"go.opentelemetry.io/otel"
@@ -216,6 +217,8 @@ func TestNewWithHeaders(t *testing.T) {
 	t.Cleanup(func() { require.NoError(t, mc.stop()) })
 
 	ctx := context.Background()
+	additionalKey := "additional-custom-header"
+	ctx = metadata.AppendToOutgoingContext(ctx, additionalKey, "additional-value")
 	exp := newGRPCExporter(t, ctx, mc.endpoint,
 		otlptracegrpc.WithHeaders(map[string]string{"header1": "value1"}))
 	t.Cleanup(func() { require.NoError(t, exp.Shutdown(ctx)) })
@@ -224,6 +227,7 @@ func TestNewWithHeaders(t *testing.T) {
 	headers := mc.getHeaders()
 	require.Regexp(t, "OTel OTLP Exporter Go/1\\..*", headers.Get("user-agent"))
 	require.Len(t, headers.Get("header1"), 1)
+	require.Len(t, headers.Get(additionalKey), 1)
 	assert.Equal(t, "value1", headers.Get("header1")[0])
 }