add more job serve metrics

1. added a metric to monitor the number of spark executors. It will help a lot for Exploration team to check the health status of job server
2. Added a meter to measure the job submission rate. It can help detect if job server is hang (when no new job submission is accepted)

There is only one instance of num-sparkExecutor Gauge for the JobManagerActor, even though it is created every time when a spark context is created. Only the first creation of the gauge returns a new instance. The following creations simply return the instance created at the first time and set its handler to sparkContext.getExec.. So, in effect, this gauge only returns the number of executors for the last spark context.