Generalize the threshold schema to support arbitrary metrics.
This is part of migrating to the new schema.
The old bug_filing_threshold and priorities.threshold will be removed once the migration is done.
Bug: None
Change-Id: I7d79b2f43a6ce7ad187fd1e75663d0629723325f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/306560
Reviewed-by: Christoffer Jansson <jansson@webrtc.org>
Commit-Queue: Beining Chen <beining@google.com>
Cr-Commit-Position: refs/heads/main@{#40145}
4206d31ae1