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.
no spaces before/after '=' in the keyword arguments of VarsEndpoint()