Implement application SLIs for database transactions

This issue tracks the efforts for defining an application-side SLI for database transactions. The first iteration will be to target sidekiq only.

We will use the max duration of all database transactions in the completed job to determine apdex success/failure. The threshold will need to be decided after gitlab-org/gitlab!147425 (merged) is deployed and data is collected.

The analysis for the values above can be found in #2863 (comment 1812373317)

Status

Rolled out (gitlab-org/gitlab!147101 (merged)) and enabled feature-flag in gprd (gitlab-org/gitlab#456667 (comment 1870624822)).