Add flag to white list container labels for prometheus metrics

[mikkeloscar]

This adds a flag white_listed_container_labels which allows specifying
a comma separated list of container labels that are allowed to be
attached to prometheus metrics. This helps limit the label space of
each data point and limits the memory used by cadvisor compared to just
add all labels from all containers which is the default.

This is somewhat similar to the store_container_labels flag but a bit
more flexible because it allows white listing some common labels e.g.
io.kubernetes.pod.name, io.kubernetes.pod.namespace etc.

Without this white listing in place we saw the memory footprint of
cadvisor explode in our production environment because users add a lot
of data in pod labels, data which is not useful for metrics but may be
useful for the individual applications.

[k8s-ci-robot]

Hi @mikkeloscar. Thanks for your PR.

I'm waiting for a google or kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[dashpole]

/ok-to-test

[dashpole]

Just a couple nits. I think this change is good overall.

[mikkeloscar]

@dashpole Thanks for the review, I addressed your comments!

[dashpole]

lgtm