feat: Add ChannelPoolSettings Getter for gRPC's ChannelProvider (#2612)

Fixes #2610 ☕️

---------

Co-authored-by: Joe Wang <[email protected]>

Author: lqiu96

gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -473,6 +473,12 @@ public Integer getMaxInboundMetadataSize() {
     return maxInboundMetadataSize;
   }
 
+  /** The configured channel pool settings used for gRPC's ChannelProvider */
+  @BetaApi("Channel pool sizing api is not yet stable")
+  public ChannelPoolSettings getChannelPoolSettings() {
+    return channelPoolSettings;
+  }
+
   @Override
   public boolean shouldAutoClose() {
     return true;