els are merged with `labelSelector` as `key in (value)`
to select the group of existing pods which pods will be taken into consideration
for the incoming pod's pod (anti) affinity. Keys that don't exist in the incoming
pod labels will be ignored. The default value is empty.
The same key is forbidden to exist in both matchLabelKeys and labelSelector.
Also, matchLabelKeys cannot be set when labelSelector isn't set.
This is a beta field and requires enabling MatchLabelKeysInPodAffinity feature gate (enabled by default).²