Expose webhook notification metrics in Grafana

Context

Related to &8628 (closed). Followup from &8628 (comment 1162358305).

Partially related to #496 (closed).

Problem

We have zero visibility over the webhook notifications feature reliability/performance on GitLab.com.

We need visibility to assess how well/reliably it performs over time and use that to decide how urgent it's to work on Webhook notifications with at-least-once delive... (&9161).

Solution

The registry is already emitting metrics around this (source). Looking at Thanos we can see registry_notifications_* metrics there. However, these are not collected/exposed.

Tasks

1. Enumerate and document the existing Prometheus metrics for webhook notifications in a new document at docs-gitlab/metrics.md. This will be the first step towards #496 (closed) as well;
2. Create new graphs on the Grafana Application detail dashboard for all metrics identified above. Place them under a new Webhook notifications row/section;
3. Identify observability gaps (are we missing any metrics to fully understand the reliability/performance of this feature?), if any, and raise followup issues.