Metrics

Tricium analyzer to check that metrics metadata are correctly formatted and contain all of the necessary information.

Currently, this analyzer can be used for UMA histogram submissions.

For each histogram, this analyzer checks:

1. There are multiple owners, to avoid a single point of failure.
2. The first owner is an individual, not a team, so there is a clear primary point of contact.

Note: We assume that histograms.xml has been autoformatted (i.e. by running python pretty_print.py in the histograms directory, which is enforced by the presubmit tooling).