fix: reset timer each time stream is opened

Author: SamuelSchaub

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

@@ -387,6 +387,8 @@ private void initialize() {
     }
 
     if (enableKeepalive) {
+      lastServerResponseTime.set(clock.nanoTime());
+      lastClientPingTime.set(-1L);
       startClientPinger();
       startServerMonitor();
     }