chore(notifications): add endpoint name label

Context

The notification system is currently being enhanced. This issue is part of the output from Discussion: notification system metrics (#888 - closed).

Implementation

Depending on if we have multiple endpoints, I think for the failure/success or RPS metrics it may be useful to also add a tag/label with the endpointName, this way we can further break the graphs out by endpointName to tell us which/when an endpoint started behaving poorly directly from the grafana dashboards

1. Update metric labels to include an endpointName per #888 (comment 1241126339)
2. Update dashboards to filter by endpoint name we can add the endpoint label as part of Improve notifications metrics dashboard (gitlab-com/runbooks#106 - closed)