Add application metrics to build apdex scores applicable to our redis instances

As part of the service maturity review of Redis, we're reviewing the obstacles to uprgade the maturity of our redis services. One of the current absences are apdex scores:

The only shard that has a component-defined apdex score is redis-cache, which relies on the gitlab_cache_operation_duration_seconds application metric. We should add similar metrics to the application that allow us to build an apdex score for the rest of the shards and better measure the actual impact of each shards performance.