Export Jetty stats to Prometheus

[merlimat]

Motivation

Expose Jetty stats for broker and proxy directly into Prometheus. The new metrics are :

jetty_requests_total 15.0
jetty_requests_active 0.0
jetty_requests_active_max 1.0
jetty_request_time_max_seconds 0.073
jetty_request_time_seconds_total 0.147
jetty_dispatched_total 15.0
jetty_dispatched_active 0.0
jetty_dispatched_active_max 1.0
jetty_dispatched_time_max 73.0
jetty_dispatched_time_seconds_total 0.147
jetty_async_requests_total 0.0
jetty_async_requests_waiting 0.0
jetty_async_requests_waiting_max 0.0
jetty_async_dispatches_total 0.0
jetty_expires_total 0.0
jetty_responses_total{code="1xx",} 0.0
jetty_responses_total{code="2xx",} 14.0
jetty_responses_total{code="3xx",} 0.0
jetty_responses_total{code="4xx",} 1.0
jetty_responses_total{code="5xx",} 0.0
jetty_stats_seconds 179.302
jetty_responses_bytes_total 137995.0