feat(bigquery): add opentelemetry W3C Trace Context to headers (#12203)

This adds header value using OpenTelemetry's [W3C Trace Context
Propagation](https://www.w3.org/TR/trace-context-1/#trace-context-http-headers-format).

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: ldetmer

java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/telemetry/HttpTracingRequestInitializer.java

@@ -23,6 +23,8 @@
 import io.opentelemetry.api.common.AttributeKey;
 import io.opentelemetry.api.trace.Span;
 import io.opentelemetry.api.trace.Tracer;
+import io.opentelemetry.api.trace.propagation.W3CTraceContextPropagator;
+import io.opentelemetry.context.Context;
 import java.io.IOException;
 
 /**
@@ -76,6 +78,10 @@ public void initialize(HttpRequest request) throws IOException {
       // No active span to exists, skip instrumentation
       return;
     }
+    // propagate the W3C Trace Context (traceID and spanID) from the active span in headers
+    W3CTraceContextPropagator.getInstance()
+        .inject(Context.current(), request.getHeaders(), HttpHeaders::set);
+
     addInitialHttpAttributesToSpan(span, request);
 
     HttpResponseInterceptor originalInterceptor = request.getResponseInterceptor();

java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/telemetry/HttpTracingRequestInitializerTest.java

@@ -145,6 +145,23 @@ public void testNoSpanIsCreatedIfNoActiveSpan() throws IOException {
     verify(delegateInitializer, times(1)).initialize(any(HttpRequest.class));
   }
 
+  @Test
+  public void testTraceContextIsPropagatedInHeaders() throws IOException {
+    HttpTransport transport = createTransport();
+    HttpRequest request = buildGetRequest(transport, initializer, BASE_URL);
+
+    HttpResponse response = request.execute();
+    response.disconnect();
+
+    assertEquals(
+        String.format(
+            "00-%s-%s-%s",
+            parentSpan.getSpanContext().getTraceId(),
+            parentSpan.getSpanContext().getSpanId(),
+            parentSpan.getSpanContext().getTraceFlags().asHex()),
+        request.getHeaders().get("traceparent"));
+  }
+
   @Test
   public void testDelegateInitializerIsCalled() throws IOException {
     HttpRequestInitializer delegateInitializer = mock(HttpRequestInitializer.class);