Custom Metrics

[goroutinegrind]

Feature:

A way to include metrics emitted from custom modules in the prometheus registry

Backstory:

I have a requirement for cosmo router to include my own custom metrics in the prometheus registry, as well as the out-of-the-box metrics - all to be hosted on the same port.

The custom metrics are to be used from the custom modules, so ideally it would be something we can instantiate a global registry and pass it where required.

This was already possible previously via replacing Main() with a custom implementation of the router startup and using the core.WithMetrics() option, to override the Prometheus.TestRegistry with the Global registry. Albeit, it was not ideal to use something called TestRegistry, however on inspection, it was only called that due to being used for tests only. It works for the above stated purpose.

Proposal:

I propose

1. TestRegistry can be renamed to Registry (or something more suitable) since it is not only viable to be used for tests
2. To allow a way to inject router options from the Main() entrypoint - since some options are not capable of being configured via the config file, and creating these things inside the router, means they are not available to custom modules. Alternatively, if there was a way to expose such things to custom modules, that would a potential solution as well.