WDYT about tsmon or BQ integration? This way we don't need to dial back. We may be able to go w/o a deprecated field, though it could be useful to warn potential new users of a group that we are trying to delete.

By tsmon you mean introduce a counter "used_in_lookups" with metric field "group"? That's a good idea. Two concerns:
  * I'm slightly worried about performance, tsmon metric implementation is kind of slow if metrics are updated from tight loops (and group look up code is pretty tight).
  * For infrequently used groups we may miss single monitoring blip. It can get eaten by retention policies before we notice it.

I think BQ in a form of eventmon is too heavy for this (we'll have to vendor a lot of libraries everywhere, then maintain ACLs and consistently use BQ table schema across all apps). Dialing back is simpler.

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