Automatic merge from submit-queue (batch tested with PRs 67397, 68019). If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.
Fix conversion for autoscaling/v1 ObjectMetricSource and add fuzzer
**What this PR does / why we need it**:
Selectors in ObjectMetricSource's weren't being persisted through roundtrip conversions, and this wasn't caught because we had no fuzzer testing MetricIdentifier selectors
**Which issue(s) this PR fixes**:
none
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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