Add support for specifying 'Gatherer' in prometheus Reporter options. (#80)
Add support for specifying 'Gatherer' in prometheus Reporter options. (#80)

Passing the Gatherer is required to be able return the HTTPHandler
appropriate for a customer Registerer that is passed in. Without this
change, the HTTPHandler is *always* associated with the Gatherer for the
promhttp.DefaultRegisterer, which is confusing if a custom Registerer is
passed.
uber-go/tally,uber-go/tally