Read mozilla-central's metrics_index instead of requiring manual addition of new definitions files

[chutten]

Presently we order Firefox on Glean consumers to manually file bugs whenever they add new metrics.yaml or pings.yaml files to metrics_index.py.

This isn't the most ergonomic way to deal with this. Could we have probe-scraper pull https://hg.mozilla.org/mozilla-central/file/tip/toolkit/components/glean/metrics_index.py and read the list of yamls from there (like mozilla-central's build does)?