Persist performance metrics for a Merge Request within GitLab

Description

As part of gitlab-ce#26944, we are displaying spark lines in Merge Requests for projects and deploys where we can track them. Prometheus however does not retain data for a long period of time, and this data will become unavailable. (For example after 30 days.)

We should therefore persist this snippet of data within GitLab, so that it can be displayed after the Prometheus server retention period has expired. We should collect it as soon as the time window for the sparkline ends. (For example, 30 minutes after merge).

Proposal

Links / references

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