Make webrick logger configurable

[shouichi]

Background:

We run containers and collect their stdout/stderr using a log collecting agent. The agent marks stderr as error logs. Yabeda::Prometheus::Exporter internally uses Rack::CommonLogger without passing logger. In this case Rack::CommonLogger logs to stderr. As a result, simple webrick logs are marked as errors in our environment.

Possible solutions:

• Enable to pass logger to Rack::CommonLogger
• Enable to configure the entire rack app

Thanks in advance.

[Envek]

Fixed in #19 and released in 0.8.0 (please note that logging now is disabled by default)

[shouichi]

Thank you!