Add a Dangerbot rule to suggest adding specs for Redis/HLL metrics migration

What does this MR do and why?

This MR adds a simple warning if new keys were added to Redis counters override files. The metric migrations are pretty much done and this is just to ensure we wont forget add specs for the rest of the keys (its error prone to test correct keys correlation manually each time)

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See an example MR !161419 (closed)

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How to set up and validate locally

Related to #476220 (closed)