chore(x-goog-spanner-request-id): set span attribute x_goog_spanner_request_id

This sets the span attribute x_goog_spanner_request_id so as to enable
correlation with distributed tracing with OpenTelemetry.

Updates #3537

Author: odeke-em

google-cloud-spanner/src/main/java/com/google/cloud/spanner/XGoogSpannerRequestId.java

@@ -159,4 +159,8 @@ public XGoogSpannerRequestId nextRequestId(long channelId, int attempt) {
       return XGoogSpannerRequestId.of(1, 1, 1, 0);
     }
   }
+
+  public static String getRequestIdFromMetadata(Metadata md) {
+    return md.get(REQUEST_HEADER_KEY);
+  }
 }

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/HeaderInterceptor.java

@@ -28,6 +28,7 @@
 import com.google.cloud.spanner.CompositeTracer;
 import com.google.cloud.spanner.SpannerExceptionFactory;
 import com.google.cloud.spanner.SpannerRpcMetrics;
+import com.google.cloud.spanner.XGoogSpannerRequestId;
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
 import com.google.spanner.admin.database.v1.DatabaseName;
@@ -174,6 +175,10 @@ private void processHeader(
         }
         if (span != null) {
           span.setAttribute("gfe_latency", String.valueOf(gfeLatency));
+          String reqId = XGoogSpannerRequestId.getRequestIdFromMetadata(metadata);
+          if (reqId != null) {
+            span.setAttribute("x_goog_spanner_request_id", reqId);
+          }
         }
       } else {
         measureMap.put(SPANNER_GFE_HEADER_MISSING_COUNT, 1L).record(tagContext);