Provide Open Metrics (/Prometheus) Wrapper

[mxinden]

By design rust-libp2p does not expose Prometheus style metrics. Instead, given its event-based architecture, one should record the events emitted by rust-libp2p as a user.

Consider providing a wrapper around libp2p-swarm which records emitted events before bubbling them upwards. In case a generic wrapper is not feasible, offer a tutorial on how to instrument ones rust-libp2p instance.

The Prometheus format has been standardized, in a backwards compatible way, under the Open Metrics name, thus supporting Open Metrics instead of the old Prometheus style format is both more future-proof and likely better supported by monitoring systems.

Libraries:

• open-metrics-client: https://github.com/mxinden/rust-open-metrics-client
• prometheus: https://github.com/tikv/rust-prometheus/