Prefer most recent start time on duplicate Pod metrics

[dippynark]

What this PR does / why we need it:

There are situations when Metrics Server can scrape duplicate Pod metrics from different nodes (e.g. kubelet metrics have become stale on a particular node due to some garbage collection bug). In this case Metrics Server ignores the duplicate:

metrics-server/pkg/scraper/scraper.go

Lines 171 to 174 in 78192ed

	if _, podFind := res.Pods[podRef]; podFind {
	klog.ErrorS(nil, "Got duplicate pod point", "pod", klog.KRef(podRef.Namespace, podRef.Name))
	continue
	}

However, because Metrics Server scrapes Pod metrics from each node in parallel, the metrics that we end up using for calculating utilisation is fairly random and can change from scrape to scrape due to small request latency differences. This can lead to utilisation changing dramatically from scrape to scrape (e.g. in the case of CPU we get utilisation of 0 if we scrape the stale node first twice in a row, or we don't get any metrics at all if we scrape the current node and then the stale node due to this PR).

Instead, this PR uses the metrics corresponding to the Pod with the latest container start time.

I am running this PR in our dev environment and it is allowing Metrics Server to carry on providing accurate information despite kubelet serving stale metrics.

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[dippynark]

/assign @RainbowMango

[dippynark]

/assign @stevehipwell

[dippynark]

/unassign @RainbowMango