Add current count to scaling event

[cgbaker]

Issue

Add the current count to api.ScalingEvent. This allows the UI to graph the current count for alongside the target count, since this information is not easily accessible via the Nomad API.

Scaling events will happen extant jobs and validation will have ensured that the target group is valid, so this should be a (non-optional) int64 filled out by Nomad during the call to PUT /v1/.../scale, based on the current value for group.Count in the job.

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.