[Feature Request] Allow users to subscribe for metrics/pings expiration

[Dexterp37]

From this proposal

The extra-mile: enabling stakeholders to subscribe for expiration notifications
The Glean Dictionary has proven to be an invaluable tool to support Glean metrics discovery and annotation with its user-friendly experience. This one-stop-shop could be potentially extended to enable users to subscribe for changes to specific metrics, for example its expiration. This would help with problem (A) since people who are interested in the data are usually not the ones that instrumented the metric in the first place, and thus their email address doesn’t usually get notified when a metric is about to expire: they just find out that the data is missing in the worst possible time, when the data is required.
Such a feature would require some design, but could potentially bring huge benefits.

This could potentially be implemented using e mechanism similar to the Glean annotations, e.g. having users edit a file in the annotations repository to add their email address, and then having the probe-scraper to parse that.