perf(spanner): avoid cloning metadata map in metrics interceptor

handwritten/spanner/src/metrics/interceptor.ts

@@ -1,4 +1,4 @@
-// Copyright 2025 Google LLC
+// Copyright 2025 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -48,7 +48,11 @@ export const MetricInterceptor = (options, nextCall) => {
           // GFE/AFE latency if available,
           // or else increase the GFE/AFE connectivity error count
           if (metricsTracer) {
-            const serverTimingHeader = metadata.getMap()['server-timing'];
+            const serverTimingEntries = metadata.get('server-timing');
+            const serverTimingHeader =
+              serverTimingEntries.length > 0
+                ? String(serverTimingEntries[0])
+                : undefined;
             const gfeTiming =
               metricsTracer?.extractGfeLatency(serverTimingHeader);
             metricsTracer.gfeLatency = gfeTiming ?? null;

handwritten/spanner/test/metrics/interceptor.ts

@@ -1,4 +1,4 @@
-// Copyright 2025 Google LLC
+// Copyright 2025 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -25,13 +25,13 @@
   let mockMetricsTracer: sinon.SinonStubbedInstance<MetricsTracer>;
   let mockFactory: sinon.SinonStubbedInstance<MetricsTracerFactory>;
   let mockNextCall: sinon.SinonStub;
   let mockInterceptingCall: any;
   let mockListener: any;
   let serverTimingMetadata: any;
   let emptyMetadata: any;
   let mockStatus: any;
   let mockOptions: any;
   let capturedListener: any;
   let testMetadata: grpc.Metadata;
 
   beforeEach(() => {
@@ -214,5 +214,18 @@
         Status.OK,
       );
     });
+
+    it('reads server-timing header using metadata.get without calling metadata.getMap', () => {
+      const getMapSpy = sandbox.spy(serverTimingMetadata, 'getMap');
+      const getSpy = sandbox.spy(serverTimingMetadata, 'get');
+      const interceptingCall = MetricInterceptor(mockOptions, mockNextCall);
+      interceptingCall.start(testMetadata, mockListener);
+
+      capturedListener.onReceiveMetadata(serverTimingMetadata);
+
+      assert.strictEqual(getMapSpy.callCount, 0);
+      assert.strictEqual(getSpy.calledWith('server-timing'), true);
+      assert.strictEqual(mockMetricsTracer.extractGfeLatency.calledOnce, true);
+    });
   });
 });