set initial pool size to 0 make dynamic channel pool work

Author: rahul2393

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

@@ -640,7 +640,7 @@ private static void maybeEnableGrpcGcpExtension(
     // When disabled, use the explicitly configured numChannels.
     final int poolSize =
         options.isDynamicChannelPoolEnabled()
-            ? options.getDynamicPoolInitialSize()
+            ? 0
             : options.getNumChannels();
 
     ApiFunction<ManagedChannelBuilder, ManagedChannelBuilder> apiFunction =