feat: add first byte latency metric (GoogleCloudPlatform#9754)

NOTE: this depends on and includes https://togithub.com/GoogleCloudPlatform/java-docs-samples/pull/9753

Author: igorbernstein2

bigtable/bigtable-proxy/src/main/java/com/google/cloud/bigtable/examples/proxy/metrics/Metrics.java

@@ -41,6 +41,8 @@ public interface Metrics {
 
   void recordCallLatency(MetricsAttributes attrs, Status status, Duration duration);
 
+  void recordFirstByteLatency(MetricsAttributes attrs, Duration duration);
+
   void updateChannelCount(int delta);
 
   interface MetricsAttributes {}

bigtable/bigtable-proxy/src/main/java/com/google/cloud/bigtable/examples/proxy/metrics/MetricsImpl.java

@@ -87,6 +87,7 @@ public class MetricsImpl implements Closeable, Metrics {
   private final DoubleHistogram clientCredLatencies;
   private final DoubleHistogram clientQueueLatencies;
   private final DoubleHistogram clientCallLatencies;
+  private final DoubleHistogram clientCallFirstByteLatencies;
   private final LongCounter serverCallsStarted;
   private final LongHistogram requestSizes;
   private final LongHistogram responseSizes;
@@ -195,6 +196,13 @@ private static SdkMeterProvider createMeterProvider(Credentials credentials, Str
             .setUnit("ms")
             .build();
 
+    clientCallFirstByteLatencies =
+        meter
+            .histogramBuilder(METRIC_PREFIX + "client.first_byte.duration")
+            .setDescription("Latency from start of request until first response is received")
+            .setUnit("ms")
+            .build();
+
     channelCounter =
         meter
             .upDownCounterBuilder(METRIC_PREFIX + "client.channel.count")
@@ -306,6 +314,11 @@ public void recordCallLatency(MetricsAttributes attrs, Status status, Duration d
     numOutstandingRpcs.decrementAndGet();
   }
 
+  @Override
+  public void recordFirstByteLatency(MetricsAttributes attrs, Duration duration) {
+    clientCallFirstByteLatencies.record(duration.toMillis(), unwrap(attrs));
+  }
+
   @Override
   public void updateChannelCount(int delta) {
     channelCounter.add(delta);

bigtable/bigtable-proxy/src/main/java/com/google/cloud/bigtable/examples/proxy/metrics/Tracer.java

@@ -106,6 +106,14 @@ public void inboundHeaders(Metadata headers) {
             d -> metrics.recordGfeLatency(attrs, d), () -> metrics.recordGfeHeaderMissing(attrs));
   }
 
+  @Override
+  public void inboundMessage(int seqNo) {
+    if (seqNo == 0) {
+      metrics.recordFirstByteLatency(
+          attrs, Duration.ofMillis(stopwatch.elapsed(TimeUnit.MILLISECONDS)));
+    }
+  }
+
   public void onCallFinished(Status status) {
     grpcQueueDuration.ifPresent(d -> metrics.recordQueueLatency(attrs, d));
     metrics.recordResponseSize(attrs, responseSize.get());

bigtable/bigtable-proxy/src/test/java/com/google/cloud/bigtable/examples/proxy/metrics/NoopMetrics.java

@@ -51,6 +51,9 @@ public void recordGfeHeaderMissing(MetricsAttributes attrs) {}
   @Override
   public void recordCallLatency(MetricsAttributes attrs, Status status, Duration duration) {}
 
+  @Override
+  public void recordFirstByteLatency(MetricsAttributes attrs, Duration duration) {}
+
   @Override
   public void updateChannelCount(int delta) {}
 }