Add missing metric definition for "counts_monthly.promoted_issues"

Related to #340633 (closed).

Current state

#340503 (closed) revealed that dynamically-added metrics that have missing definitions are no longer included in the Service Ping payload. The following metric is currently missing:

 "counts_monthly.promoted_issues"

Proposal

Add the missing definition for the counts_monthly.promoted_issues metric to reintroduce it to the payload.