ref(grouping): Add sample rate for grouphash metadata backfill

[lobsterkatie]

This adds a new option, grouping.grouphash_metadata.backfill_sample_rate, to control the rate at which we add or update grouphash metadata for existing grouphashes. It also adds and switches to using a helper function, should_handle_grouphash_metadata, which checks both the existing killswitch and feature flag and the new option in cases where the grouphash already exists.

There's not yet any code to do the adding/updating (and the option defaults to 0), so this doesn't change any behavior*, but it does set us up for when said code is added.

*This is only 99.9% true. Technically there is a theoretical behavior change, in that the one bit of updating we already do (to the grouping config) is now subject to the sample rate, which is currently 0. That said, we've been on the same grouping config since we started collecting grouphash metadata, so there aren't any records to update. So a thing that never happens now actually can't happen (at least until we up the sample rate), but in the real world that doesn't actually make things any different.