util-stats: document metric_metadata.json endpoint
util-stats: document metric_metadata.json endpoint

Problem

The Metric Metadata Endpoint (/admin/metric_metadata.json) had no documentation, making it tough for some users to decipher the meaning of some fields.

Solution
Add a section to the Admin.html docs about the Metric Metadata Endpoint.

JIRA Issues: CSL-8320

Differential Revision: https://phabricator.twitter.biz/D639225

twitter/twitter-server,twitter/twitter-server,twitter/twitter-server