fix: add a default value for max outstanding request bytes flow control setting (#96)

* adding default max outstanding request bytes

Author: hannahrogers-google

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Subscriber.java

@@ -416,7 +416,10 @@ public static final class Builder {
     private Duration maxAckExtensionPeriod = DEFAULT_MAX_ACK_EXTENSION_PERIOD;
 
     private FlowControlSettings flowControlSettings =
-        FlowControlSettings.newBuilder().setMaxOutstandingElementCount(1000L).build();
+        FlowControlSettings.newBuilder()
+            .setMaxOutstandingElementCount(1000L)
+            .setMaxOutstandingRequestBytes(100L * 1024L * 1024L) // 100MB
+            .build();
 
     private ExecutorProvider executorProvider = DEFAULT_EXECUTOR_PROVIDER;
     private ExecutorProvider systemExecutorProvider = null;