Value Stream Request for Assigned Time: Add Start/End Event for Issue Assigned
# Description
In order to better utilize our custom value stream analytics, our Team (Belvedere Trading) wants to gain more granularity on how long it takes for an issue to be completed (ie. IP-closed).
This metric is similar to 'Cycle Time' but differs in the sense that it includes completion times of issues that may not have a merge request.
# Requirements
* Add Issue Assigned as an option in the 'Start Event' and 'End Event' Custom Value Stream Analytics
Tangentially, our Value Stream Analytics currently does not show Cycle Time. We would like to know if this is expected

Appreciate any and all feedback/help!
# Proposal
In https://gitlab.com/gitlab-org/gitlab/-/issues/397050 we will start capturing the assignment values. Once that's complete, we will add the new pairing rule (this issue):
> Add the event definition, `First assigned at` (Weight: 2)
>
> Here, the query should find the first assignment event for the given object (Issue or MR) and use the `created_at` column as the timestamp.
Copied from https://gitlab.com/gitlab-org/gitlab/-/issues/361983#note_1318526653
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