prometheus: Avoid duplicated metrics registration

Praefect uses prometheus to export metrics from inside.
It relies on the defaults from the prometheus library
to gather set of metrics and register a new metrics.
Because of it the new metrics got registered on the
DefaultRegisterer - a global pre-configured registerer.
Because of that we can't call 'run' function multiple
times (for testing purposes) as it results to the metrics
registration error. To omit that problem the 'run' function
extended with prometheus.Registerer parameter that is used
to register praefect custom metrics. The production code
still uses the same DefaultRegisterer as it was before.
And the test code creates a new instance of the registerer
for each 'run' invocation, so there are no more duplicates.