feat: resizing channel pool size based on the work load

Author: mutianf

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

@@ -24,6 +24,7 @@
 import com.google.api.gax.batching.FlowController.LimitExceededBehavior;
 import com.google.api.gax.core.FixedCredentialsProvider;
 import com.google.api.gax.core.GoogleCredentialsProvider;
+import com.google.api.gax.grpc.ChannelPoolSettings;
 import com.google.api.gax.grpc.InstantiatingGrpcChannelProvider;
 import com.google.api.gax.retrying.RetrySettings;
 import com.google.api.gax.rpc.FixedHeaderProvider;
@@ -250,7 +251,13 @@ public Map<String, String> getJwtAudienceMapping() {
   /** Returns a builder for the default ChannelProvider for this service. */
   public static InstantiatingGrpcChannelProvider.Builder defaultGrpcTransportProviderBuilder() {
     return BigtableStubSettings.defaultGrpcTransportProviderBuilder()
-        .setPoolSize(getDefaultChannelPoolSize())
+        .setChannelPoolSettings(
+            ChannelPoolSettings.builder()
+                .setInitialChannelCount(2 * Runtime.getRuntime().availableProcessors())
+                .setMinRpcsPerChannel(1)
+                .setMaxRpcsPerChannel(50)
+                .setPreemptiveRefreshEnabled(true)
+                .build())
         .setMaxInboundMessageSize(MAX_MESSAGE_SIZE)
         .setKeepAliveTime(Duration.ofSeconds(30)) // sends ping in this interval
         .setKeepAliveTimeout(