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)

Baohua, you are right. This metric is 1-to-1 correspondence for spark context. With current job server design, only one spark context is supported for each jobManager actor. So it should not be an issue at this time.