pubsub: make logs quieter (#2492)

Retryable stream closure happens quite often.
This commit lowers the log level to FINE to not spam the console.

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

@@ -229,11 +229,12 @@ public void onFailure(Throwable cause) {
               logger.log(Level.FINE, "pull failure after service no longer running", cause);
               return;
             }
-            logger.log(Level.WARNING, "Terminated streaming with exception", cause);
             if (!StatusUtil.isRetryable(cause)) {
+              logger.log(Level.SEVERE, "terminated streaming with exception", cause);
               notifyFailed(cause);
               return;
             }
+            logger.log(Level.FINE, "stream closed with retryable exception; will reconnect", cause);
             long backoffMillis = channelReconnectBackoffMillis.get();
             long newBackoffMillis =
                 Math.min(backoffMillis * 2, MAX_CHANNEL_RECONNECT_BACKOFF.toMillis());