Add instrumentation (tracing and metrics)

[ethanfrey]

Idea: monkit, opentracing, prometheus???

Something so we can get metrics from a running system, find real bottlenecks

[ethanfrey]

We seem to have some working code for existing metrics which is very helpful.

This is to define an idea of how to plug in more metric collection from the abci app for various items of interest.

I think @alpe already slack-ed some ideas on what to do, just write those down in a markdown file for now. We can implement later.

[alpe]

Exporting some KPI metrics via prometheus would be nice. Like number of wallets, Tx send, usernames registered, etc for graphs.
Also request duration, result status, etc for fine grained analysis. But we get a lot from traefik already.