This adds a statsd_metrics.yaml file that registers all the statsd
metrics that Antenna emits.

This adds a document_metrics.py Sphinx extension for autogenerating
metrics documentation.

While doing this, I removed statsd bits from health state since we don't
use that anymore.

This changes the sentry_scrub_error metric to use the
``socorro.sentry_scrub_error`` key and pass the service as a tag. This
matches what we're changing the other Socorro services to do.

This fixes the tests so they set up logging and metrics at pytest
session start so that the metrics filters apply to all the tests.

This also removes the ``CaptureMetricsUsed`` because we don't need that
anymore.