Handle nuances of "in source" better

[wlach]

Currently our concept of "in source" only considers the latest version of a product, but it's possible in some cases for a metric to be still "in source" on other channels even if it's expired there. The most common case is where a metric is removed on the nightly channel of an application, but remains on beta and/or release.

glean-dictionary/etl/__main__.py

Line 281 in ce84fc6

in_source=metric.definition["in_source"],

For applications like Firefox for Android with multiple app identifiers, this is easy to detect since metrics are segmented by app id (and are just amalgamated by our ETL). In that case we should probably not consider a metric removed on all channels unless it actually is (instead we should move to a per-channel notion of removed?).

This is more difficult for Firefox Desktop: perhaps updating the warning banner in the glean dictionary is the best we can do.

/cc @chutten

[chutten]

We could make Glean Dictionary aware of Firefox Desktop's channels in a similar means to Firefox for Android's. gecko-dev has beta and release channels we could specify in probe-scraper's repositories.yaml. I did think about it, but went for the more straightforward app declaration to keep things from sprawling.