Add metrics for total number of suspended workspaces
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MR: Pending
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## Description
Add metrics for suspended workspaces.
We will use internal event tracking and service ping metric.
The place we trigger events would be at workspace model `ee/app/models/remote_development/workspace.rb`, and we use `after_save` hook to inject event trigger. We should make sure even if this tracking method goes wrong, then overall operation still goes well (use rescue block).
For metrics, we should make one metric for tracking all available timeframes, 7d, 28d and all time.
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## Acceptance criteria
- [ ] Define internal event for tracking workspace suspended
- [ ] Trigger this event in workspace model `after_save` hook
- [ ] Make sure to use `rescue` to emit the error with tracking
- [ ] Define Service Ping metric for suspended workspaces, using above event data
## Implementation plan
NA
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