Use the Prometheus exposition format for exposing metrics

[leecalcote]

Does it make sense to instrument using https://github.com/prometheus/client_golang? // @kanishkarj

Originally posted by @leecalcote in https://github.com/layer5io/learn-layer5/pull/15/files

[kanishkarj]

Yes, it is. It's always better to follow a standardized format I guess, xD

[kanishkarj]

I'm curious if we can expose JSON response via the prometheus instrumentation libraries. I think it supports only counts and histogram.