Log write_count keys for each database config types

What does this MR do and why?

This MR tracks :write_count metrics and log keys per database. It adds db_primary_write_count and db_<main/ci>_write_count to the logs. We already track total db counts and cached counts. This allows us to track write counts per database.

Looking at gitlab-com/gl-infra/scalability#3771 (comment 2074687023), that would help us identify Sidekiq workers which are read or write-skewed on a database. That would empower devs to set data consistency more accurately.

Follow-up: open MR on runbooks index

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