fix grpc set metadata multi-value (#11308)

Co-authored-by: Lauri Tulmin <ltulmin@splunk.com>

instrumentation/grpc-1.6/library/src/main/java/io/opentelemetry/instrumentation/grpc/v1_6/MetadataSetter.java

@@ -13,6 +13,8 @@ enum MetadataSetter implements TextMapSetter<Metadata> {
 
   @Override
   public void set(Metadata carrier, String key, String value) {
-    carrier.put(Metadata.Key.of(key, Metadata.ASCII_STRING_MARSHALLER), value);
+    Metadata.Key<String> metadataKey = Metadata.Key.of(key, Metadata.ASCII_STRING_MARSHALLER);
+    carrier.removeAll(metadataKey);
+    carrier.put(metadataKey, value);
   }
 }

instrumentation/grpc-1.6/library/src/test/java/io/opentelemetry/instrumentation/grpc/v1_6/MetadataSetterTest.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.grpc.v1_6;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import io.grpc.Metadata;
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.api.trace.SpanContext;
+import io.opentelemetry.api.trace.SpanId;
+import io.opentelemetry.api.trace.TraceFlags;
+import io.opentelemetry.api.trace.TraceId;
+import io.opentelemetry.api.trace.TraceState;
+import io.opentelemetry.api.trace.propagation.W3CTraceContextPropagator;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.context.propagation.TextMapGetter;
+import io.opentelemetry.context.propagation.TextMapPropagator;
+import javax.annotation.Nullable;
+import org.junit.jupiter.api.Test;
+
+class MetadataSetterTest {
+
+  @Test
+  void overwriteTracingHeader() {
+    Metadata metadata = new Metadata();
+    TextMapPropagator propagator = W3CTraceContextPropagator.getInstance();
+    for (int i = 1; i <= 2; i++) {
+      Context context =
+          Context.root()
+              .with(
+                  Span.wrap(
+                      SpanContext.create(
+                          TraceId.fromLongs(0, i),
+                          SpanId.fromLong(i),
+                          TraceFlags.getDefault(),
+                          TraceState.getDefault())));
+      propagator.inject(context, metadata, MetadataSetter.INSTANCE);
+    }
+
+    assertThat(metadata.getAll(Metadata.Key.of("traceparent", Metadata.ASCII_STRING_MARSHALLER)))
+        .hasSize(1);
+
+    Context context =
+        propagator.extract(
+            Context.root(),
+            metadata,
+            new TextMapGetter<Metadata>() {
+              @Override
+              public Iterable<String> keys(Metadata metadata) {
+                return metadata.keys();
+              }
+
+              @Nullable
+              @Override
+              public String get(@Nullable Metadata metadata, String key) {
+                return metadata.get(Metadata.Key.of(key, Metadata.ASCII_STRING_MARSHALLER));
+              }
+            });
+
+    SpanContext spanContext = Span.fromContext(context).getSpanContext();
+    assertThat(spanContext.getTraceId()).isEqualTo(TraceId.fromLongs(0, 2));
+    assertThat(spanContext.getSpanId()).isEqualTo(SpanId.fromLong(2));
+  }
+}