Deprecate and replace `counts.merged_merge_requests_using_approval_rules` metric

What does this MR do and why?

This MR:

• Deprecates counts.merged_merge_requests_using_approval_rules according with the metric lifecycle guidelines.
• Adds a new metric counts.merged_merge_requests_using_approval_rules_distinct, which uses distinct_count using metric instrumentation classes.

@mbruemmer reported that there are some Service Ping payloads where counts.merged_merge_requests_using_approval_rules is higher than counts.merge_requests, which shouldn't be possible.

It turned out that counts.merged_merge_requests_using_approval_rules uses count instead of distinct_count to calculate the metric, with MRs being counted multiple times if they have multiple approval roles, due to the join in the query. (See the original MR for details.)

The query is:

SELECT COUNT("merge_requests"."id") FROM "merge_requests" INNER JOIN "approval_merge_request_rules" ON "approval_merge_request_rules"."merge_request_id" = "merge_requests"."id" WHERE ("merge_requests"."state_id" IN (3))

https://explain.depesz.com/s/Khqf

And it should be:

SELECT COUNT(DISTINCT "merge_requests"."id") FROM "merge_requests" INNER JOIN "approval_merge_request_rules" ON "approval_merge_request_rules"."merge_request_id" = "merge_requests"."id" WHERE ("merge_requests"."state_id" IN (3))

https://explain.depesz.com/s/JmbiU

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