fix(horizontalpodautoscaler): Added container label to ContainerResource metrics

[bxrne]

What this PR does / why we need it:

Fixes duplicate kube_horizontalpodautoscaler_spec_target_metric and kube_horizontalpodautoscaler_status_target_metric metrics when multiple containers use the same resource (e.g., CPU) in a HorizontalPodAutoscaler. I also added a unit test case to verify this fix.

Previously, when an HPA specified multiple ContainerResource metrics with the same resource name but different containers, kube-state-metrics would generate metrics with identical label sets. This caused Prometheus 2.52.0+ to log duplicate metric errors and drop samples:

msg="Error on ingesting samples with different value but same timestamp" num_dropped=12

This PR adds a container label to ContainerResource metrics, making them distinguishable:

• Before: kube_horizontalpodautoscaler_spec_target_metric{metric_name="cpu",metric_target_type="utilization"}
• After: kube_horizontalpodautoscaler_spec_target_metric{metric_name="cpu",metric_target_type="utilization",container="main"}

For example, an HPA with three containers (main, sidecar1, sidecar2) all scaling on CPU now produces three distinct metrics instead of duplicates.

How does this change affect the cardinality of KSM: increases

Cardinality increases slightly for HPAs using ContainerResource metrics. Each container adds one additional metric cardinality dimension. This is the intended behavior to distinguish per-container resource targets.

Which issue(s) this PR fixes:

Fixes #2403

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[bxrne]

@CatherineF-dev @mrueg should i mark this PR as stale or backlog, i understand its priority is low?