feat: add optional Prometheus metrics collection

[paulsturgess]

This collector can be used by apps using the yabeda and yabeda-rails gems.

For example, in initializers/apia.rb

require 'apia/prometheus_collector'

[paulsturgess]

@adamcooke @jimeh this code is lifted from Identity and putting it inside the Apia gem itself is a simple way for us to re-use it across apps. But alternatively we could create a new dedicated apia-prometheus gem. Do you have any preferences on which approach we take?

[jimeh]

@paulsturgess my preference would be a separate gem, so we don't impose metrics-specific gem dependencies on applications that don't need any metrics features out of Apia.

[paulsturgess]

my preference would be a separate gem, so we don't impose metrics-specific gem dependencies on applications that don't need any metrics features out of Apia.

OK no problem, thought you might say that. Just didn't want to go ahead and set all that up if we didn't need it. But I can go ahead and do that 👍