Client change for using request priority to enable flow control

In this stage, client needs to do 3 things:
1. Always create flow control callable regardless of client flag
2. Make sure the entire callable behave like no-op if RateLimitInfo is not present.
3. Make sure client runs flow control as long as RateLimitInfo is present, regardless of client flag.

Meanwhile, setting client flag would still set the feature flag.

On server side, we'll compute and return RateLimitInfo if server side sees priority is low.

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/EnhancedBigtableStub.java

@@ -728,19 +728,19 @@ public Map<String, String> extract(MutateRowsRequest mutateRowsRequest) {
                 .build(),
             settings.bulkMutateRowsSettings().getRetryableCodes());
 
-    ServerStreamingCallable<MutateRowsRequest, MutateRowsResponse> callable =
+    ServerStreamingCallable<MutateRowsRequest, MutateRowsResponse> statsHeader =
         new StatsHeadersServerStreamingCallable<>(base);
 
-    if (settings.bulkMutateRowsSettings().isServerInitiatedFlowControlEnabled()) {
-      callable = new RateLimitingServerStreamingCallable(callable);
-    }
+    // Always create this callable because flow control will be enabled by the presence of RateLimitInfo, not the client flag
+    ServerStreamingCallable<MutateRowsRequest, MutateRowsResponse> flowControl = new RateLimitingServerStreamingCallable(
+        statsHeader);
 
     // Sometimes MutateRows connections are disconnected via an RST frame. This error is transient
     // and
     // should be treated similar to UNAVAILABLE. However, this exception has an INTERNAL error code
     // which by default is not retryable. Convert the exception so it can be retried in the client.
     ServerStreamingCallable<MutateRowsRequest, MutateRowsResponse> convertException =
-        new ConvertExceptionCallable<>(callable);
+        new ConvertExceptionCallable<>(flowControl);
 
     ServerStreamingCallable<MutateRowsRequest, MutateRowsResponse> withBigtableTracer =
         new BigtableTracerStreamingCallable<>(convertException);

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/EnhancedBigtableStubSettings.java

@@ -975,6 +975,7 @@ public EnhancedBigtableStubSettings build() {
         this.setTransportChannelProvider(channelProviderBuilder.build());
       }
 
+      // Will be deprecated once we migrate flow control user to use request priority
       if (this.bulkMutateRowsSettings().isServerInitiatedFlowControlEnabled()) {
         // only set mutate rows feature flag when this feature is enabled
         featureFlags.setMutateRowsRateLimit(true);

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/RateLimitingServerStreamingCallable.java

@@ -31,6 +31,7 @@
 import com.google.common.base.Stopwatch;
 import com.google.common.util.concurrent.RateLimiter;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -64,6 +65,10 @@ class RateLimitingServerStreamingCallable
   // as the server side cap
   private static final double MAX_FACTOR = 1.3;
 
+  // Disabled by default, enabled if RateLimitInfo is present, which is set on server side when
+  // feature flag is present or low request priority is used.
+  private volatile boolean rateLimitEnabled = false;
+
   private final RateLimiter limiter;
 
   private final AtomicReference<Instant> lastQpsChangeTime = new AtomicReference<>(Instant.now());
@@ -81,32 +86,27 @@ public void call(
       MutateRowsRequest request,
       ResponseObserver<MutateRowsResponse> responseObserver,
       ApiCallContext context) {
-    Stopwatch stopwatch = Stopwatch.createStarted();
-    limiter.acquire();
-    stopwatch.stop();
-    if (context.getTracer() instanceof BigtableTracer) {
-      ((BigtableTracer) context.getTracer())
-          .batchRequestThrottled(stopwatch.elapsed(TimeUnit.MILLISECONDS));
+    if (rateLimitEnabled) {
+      Stopwatch stopwatch = Stopwatch.createStarted();
+      limiter.acquire();
+      stopwatch.stop();
+      if (context.getTracer() instanceof BigtableTracer) {
+        ((BigtableTracer) context.getTracer())
+            .batchRequestThrottled(stopwatch.elapsed(TimeUnit.MILLISECONDS));
+      }
     }
     RateLimitingResponseObserver innerObserver =
-        new RateLimitingResponseObserver(limiter, lastQpsChangeTime, responseObserver);
+        new RateLimitingResponseObserver(responseObserver);
     innerCallable.call(request, innerObserver, context);
   }
 
   class RateLimitingResponseObserver extends SafeResponseObserver<MutateRowsResponse> {
     private final ResponseObserver<MutateRowsResponse> outerObserver;
-    private final RateLimiter rateLimiter;
-
-    private final AtomicReference<Instant> lastQpsChangeTime;
 
     RateLimitingResponseObserver(
-        RateLimiter rateLimiter,
-        AtomicReference<Instant> lastQpsChangeTime,
         ResponseObserver<MutateRowsResponse> observer) {
       super(observer);
       this.outerObserver = observer;
-      this.rateLimiter = rateLimiter;
-      this.lastQpsChangeTime = lastQpsChangeTime;
     }
 
     @Override
@@ -116,7 +116,11 @@ protected void onStartImpl(StreamController controller) {
 
     @Override
     protected void onResponseImpl(MutateRowsResponse response) {
+      // Must not limit rate if RateLimitInfo is not present.
+      // Must limit rate and update QPS if RateLimitInfo is present, regardless of client side flag
+      // setting.
       if (response.hasRateLimitInfo()) {
+        rateLimitEnabled = true;
         RateLimitInfo info = response.getRateLimitInfo();
         // RateLimitInfo is an optional field. However, proto3 sub-message field always
         // have presence even thought it's marked as "optional". Check the factor and
@@ -126,6 +130,9 @@ protected void onResponseImpl(MutateRowsResponse response) {
               info.getFactor(),
               Duration.ofSeconds(com.google.protobuf.util.Durations.toSeconds(info.getPeriod())));
         }
+      } else {
+        // Disable in case customer switched from low to higher priorities.
+        rateLimitEnabled = false;
       }
     }