v0.6.0 Beta

Adds support for Prometheus observability (#33)

* adds basic structure for instrumenting

* cleans up the observer package a bit

* adds wiring for prometheus stub

* adds support for histograms

* adds a store Observer struct

* cleans up the store Observer usage

* starts work on broker observer

* adds observer instrimentations to the broker

* implements the combine method of observers

* exposes binding your own prometheus registerer

* fixes prometheus names