Add metric for active workspaces

What does this MR do and why?

Add event for tracking active workspces, and use label disguising started and re-started.

Add metrics for active workspace, including both started and restarted combined.

References

Issue - Add metrics for total number of active workspaces (#537570 - closed)

Screenshots or screen recordings

Workspace started the first time, and see event tracked with started label

Same workspace stopped and restarted, and see event tracked with restarted label

How to set up and validate locally

• Follow steps for local workspace setup, https://gitlab.com/gitlab-org/workspaces/gitlab-workspaces-docs/-/blob/main/doc/local-development-environment-setup.md?ref_type=heads
• Follow steps for local snowplow micro setup, https://docs.gitlab.com/development/internal_analytics/internal_event_instrumentation/local_setup_and_debugging/#snowplow-micro
• Go to workspace creation page http://gdk.test:3000/-/remote_development/workspaces/new, and create one workspace, and see event tracked with started label when workspace is running
• On same page, stop the same workspace first, and then restart the workspace, and see event tracked with restarted label

MR acceptance checklist

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