Instrument the Atlas HTTP Client RoundTripper

[Freyert]

I've added a special Error type HTTPError in #14. This was outside the scope of the PR and therefore I didn't not focus on doing this correctly. Sometimes doing things the inefficiently helps us learn to do things more efficiently.

Instead of handling/counting each error code whenever they occur in the code it is much better practice to wrap the RoundTripper for an HTTPClient. This wrapping acts as a sort of middleware from which we can instrument all HTTP calls at a single point.

Specifically we need to wrap the round tripper with InstrumentRoundTripperCounter. It will partition the CounterVec by method and code. This is exactly what we want.

The prometheus team has provided an example in their tests:
https://github.com/prometheus/client_golang/blob/0400fc44d42dd0bca7fb16e87ea0313bb2eb8c53/prometheus/promhttp/instrument_client_test.go#L204-L210