Adding Metrics / Logging

## What is your idea?

Follow up from https://github.com/aws/event-ruler/pull/18. Added visibility around ruler internals would be helpful when debugging issues or building / updating the code. We want to do this without taking any unnecessary dependency (esp for logging / metrics, where clients can be very opinionated on what they prefer).

## Would you be willing to make the change?

## Additional context

I'll paste relevant github PR discussions as snippets as comments.

Start by reading the discussion in pull request #18 and the relevant issue comments, then inspect the ruler internals to identify what visibility is needed. Define a dependency-free metrics and logging approach; done means the agreed debugging visibility is available without imposing a client logging or metrics dependency.

Written by the indexing model from the issue text.