Add prometheus section to config
Add prometheus section to config

We won't expose Prometheus metrics by default. Those who want to opt
into them can add a section to their config like:

    [prometheus]
    enabled = true

    [prometheus.labels]
    foo = bar
    spam = eggs

Prometheus metrics are exposed by default on the /metrics endpoint.
Change this by setting an 'endpoint' key.

deniszh/go-carbon,lomik/go-carbon,deniszh/go-carbon,deniszh/go-carbon,lomik/go-carbon,lomik/go-carbon