Merge pull request #60886 from mattjmcnaughton/mattjmcnaughton/59975-object-metrics-ignore-unready-pods

Automatic merge from submit-queue. 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>.

Ignore unready pods when calculating desired replicas

**What this PR does / why we need it**:

This PR causes `GetExternalMetricReplicas` and `GetObjectMetricReplicas` to ignore unready pods when computing the number of desired replicas. If we don't ignore unready pods, there is a risk of overscaling. See the commit messages for examples and implementation info.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #59975 

**Special notes for your reviewer**:
@MaciekPytel and I consciously chose to save `GetExternalPerPodMetricReplicas` for a separate PR, as we aren't definite on what is the preferred behavior.

**Release note**:

```release-note
Unready pods will no longer impact the number of desired replicas when using horizontal auto-scaling with external metrics or object metrics.
```