Improve handling of deployments that are not instantaneous on MR widget
As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/40609#note_50529332, we were previously considering deployments to be instantaneous once they were considered "finished".
This is a bit naive, as of course deployments take time to fully roll out, especially in production environments.
To better handle this, we should detect both the start time and the end time of the deployment job. The start time is currently not easily available, but is being added in https://gitlab.com/gitlab-org/gitlab-ce/issues/40930.
Once this data is available, we should update the MR Prometheus widget:
- Previous version is tracked up until the deploy job starts.
- New version begins when the deploy job ends.
We could then make a visual indicator, maybe widening the bar or something, to indicate that the deploy was "in progress" during that timeframe.