feat(spanner): set manual affinity incase of gRPC-GCP extenstion (#3215)

* feat(spanner): set manual affinity incase of gRPC-GCP extenstion

* feat(spanner): convert channel hint to be bounded

* feat(spanner): check if extension is enabled

Author: harshachinta

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

@@ -56,6 +56,7 @@
 import com.google.api.pathtemplate.PathTemplate;
 import com.google.cloud.RetryHelper;
 import com.google.cloud.RetryHelper.RetryHelperException;
+import com.google.cloud.grpc.GcpManagedChannel;
 import com.google.cloud.grpc.GcpManagedChannelBuilder;
 import com.google.cloud.grpc.GcpManagedChannelOptions;
 import com.google.cloud.grpc.GcpManagedChannelOptions.GcpMetricsOptions;
@@ -267,6 +268,8 @@ public class GapicSpannerRpc implements SpannerRpc {
   private static final ConcurrentMap<String, RateLimiter> ADMINISTRATIVE_REQUESTS_RATE_LIMITERS =
       new ConcurrentHashMap<>();
   private final boolean leaderAwareRoutingEnabled;
+  private final int numChannels;
+  private final boolean isGrpcGcpExtensionEnabled;
 
   public static GapicSpannerRpc create(SpannerOptions options) {
     return new GapicSpannerRpc(options);
@@ -318,6 +321,8 @@ public GapicSpannerRpc(final SpannerOptions options) {
     this.callCredentialsProvider = options.getCallCredentialsProvider();
     this.compressorName = options.getCompressorName();
     this.leaderAwareRoutingEnabled = options.isLeaderAwareRoutingEnabled();
+    this.numChannels = options.getNumChannels();
+    this.isGrpcGcpExtensionEnabled = options.isGrpcGcpExtensionEnabled();
 
     if (initializeStubs) {
       // First check if SpannerOptions provides a TransportChannelProvider. Create one
@@ -1960,7 +1965,20 @@ <ReqT, RespT> GrpcCallContext newCallContext(
       boolean routeToLeader) {
     GrpcCallContext context = GrpcCallContext.createDefault();
     if (options != null) {
-      context = context.withChannelAffinity(Option.CHANNEL_HINT.getLong(options).intValue());
+      if (this.isGrpcGcpExtensionEnabled) {
+        // Set channel affinity in gRPC-GCP.
+        // Compute bounded channel hint to prevent gRPC-GCP affinity map from getting unbounded.
+        int boundedChannelHint = Option.CHANNEL_HINT.getLong(options).intValue() % this.numChannels;
+        context =
+            context.withCallOptions(
+                context
+                    .getCallOptions()
+                    .withOption(
+                        GcpManagedChannel.AFFINITY_KEY, String.valueOf(boundedChannelHint)));
+      } else {
+        // Set channel affinity in GAX.
+        context = context.withChannelAffinity(Option.CHANNEL_HINT.getLong(options).intValue());
+      }
     }
     if (compressorName != null) {
       // This sets the compressor for Client -> Server.