feat(bigtable): lower the value for max rpc channels to make it more conservative

Author: sushanb

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

@@ -446,7 +446,9 @@ public static InstantiatingGrpcChannelProvider.Builder defaultGrpcTransportProvi
             ChannelPoolSettings.builder()
                 .setInitialChannelCount(10)
                 .setMinRpcsPerChannel(1)
-                .setMaxRpcsPerChannel(50)
+                // Keep it conservative as we scale the channel size every 1min
+                // and delta is 2 channels. 
+                .setMaxRpcsPerChannel(25)
                 .setPreemptiveRefreshEnabled(true)
                 .build())
         .setMaxInboundMessageSize(MAX_MESSAGE_SIZE)