rpc: metrics for JSON-RPC method calls (#20847)

This adds a couple of metrics for tracking the timing
and frequency of method calls:

- rpc/requests gauge counts all requests
- rpc/success gauge counts requests which return err == nil
- rpc/failure gauge counts requests which return err != nil
- rpc/duration/all timer tracks timing of all requests
- rpc/duration/<method>/<success/failure> tracks per-method timing