Add metrics support

[jhalterman]

Add support for instrumenting different policies with metrics. This instrumentation could live in a separate package, such as how the HTTP support works, and can instrument policies using their event listeners.

[gaby]

Is this adding support using Prometheus or OpenTelemetry?

[amanhigh]

In actual Hystrix, there was also a Hystrix dashboard which was more real-time, but open telemetry would also work.