Close #2992 

Breaking changes:
- rpc server grafana metric `substrate_rpc_requests_started` is removed
(not possible to implement anymore)
- rpc server grafana metric `substrate_rpc_requests_finished` is removed
(not possible to implement anymore)
- rpc server ws ping/pong not ACK:ed within 30 seconds more than three
times then the connection will be closed

Added
- rpc server grafana metric `substrate_rpc_sessions_time` is added to
get the duration for each websocket session