Expose more metrics to improve observability:

Montior API:
  - Add `retained_msg_count` field to `/api/v5/monitor_current`.
  - Add `retained_msg_count` and `node_uptime` fields to `/api/v5/monitor_current/nodes/{node}`.

Prometheus API:
  - Add `emqx_cert_expiry_at` to `/api/v5/prometheus/stats` to display TLS listener certificate expiration time.
  - Add `/api/v5/prometheus/auth` endpoint to provide metrics such as execution count and running status for all authenticatiors and authorizators.
  - Add `/api/v5/prometheus/data_integration` endpoint to provide metrics such as execution count and status for all rules, actions, and connectors.

Limitations:
  Prometheus push gateway only supports content in `/api/v5/prometheus/stats?mode=node` for now.

For more API details and metric type information. Please see also in swagger api docs.