Follow-up from "Add enforcement_type label to MR approval policy metrics"

The following discussion from !215825 (merged) should be addressed:

• @michold started a discussion:

  non-blocking: We can make this notation a bit simpler:

            .and increment_usage_metrics(
              'redis_hll_counters.count_distinct_namespace_id_from_applied_enforce_merge_request_approval_policies_monthly', # rubocop disable Layout/LineLength
              'redis_hll_counters.count_distinct_project_id_from_applied_enforce_merge_request_approval_policies_monthly'
            )