cleanup: fix typo and delete useless parameter

[nayihz]

There are three changes in this PR:

• delete metricsStalenessThreshold field in PodMetricsFactory struct and also delete this in NewPodMetricsFactory func
• rename AllPodPredicate to AllPodsPredicate to fix typo
• add PodsWithFreshMetrics func to initialize a predicate function for PodList

/cc @liu-cong @nirrozenbaum

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