Automatic merge from submit-queue (batch tested with PRs 63349, 63294). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
Add metric for throttled requests in AWS
**What this PR does / why we need it**:
This PR adds a metric for request throttling in AWS.
**Special notes for your reviewer**:
* Added metric.
* Moved metrics-related code to `aws_metrics.go`.
* Capitalized acronyms, e.g., `recordAwsMetric` to `recordAWSMetric`.
**Release note**:
```release-note
NONE
```
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