Add filtering for py metrics

Metrics can be expensive to store. Not every metric is needed: some metrics aren't needed after the incident remediations were put in place, other metrics are feature flags. Ostrich and Twitter-Server already have the functionality to do runtime filtering of metrics according to a preconfigured blacklist of regexes. This means that by passing "filtered=1", it can filter out a subset of metrics, whereas without this parameter, all metrics would be returned.

This adds similar abilities to common metrics for Python.

See internal reviewboard 380359.
rather than making comma reserved, you could just do default=[], action='append' and then allow the user to specify the flag multiple times, e.g. --vars-stats-filter foo --vars-stats-filter bar