e2e: Use metrics container when executing commands

[mkowalski]

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This PR adds explicit -c nmstate-metrics to the command invocations that use kubectl.sh exec [...]. This should not change the current behaviour as by default we execture them in this container, however it's been noticed that the following stderr is produced

stderr: Defaulted container "nmstate-metrics" out of: nmstate-metrics, kube-rbac-proxy

so for being explict, we are defining where to execute the command.

Release note:

NONE