Add `ScanPipelineService` execution duration histogram and dashboard

Why are we doing this work

We need to measure the execution time of ScanPipelineService.

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• backend add a Prometheus histogram with suitable buckets and labelled by the # actions and the project_id the service executes for (draft)
• backend add a heatmap panel for the metric to our group's Grafana dashboard (see this MR for an example)

Verification steps