Add flowcontrol queue length in bytes metric

[RyanRosario]

What type of PR is this?
/kind documentation
/kind feature

What this PR does / why we need it:

It adds new observability metrics for flow control.

Which issue(s) this PR fixes:
Related to #1708

Does this PR introduce a user-facing change?:

NO

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: RyanRosario
Once this PR has been reviewed and has the lgtm label, please assign danehans for approval. For more information see the Code Review Process.

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[k8s-ci-robot]

Hi @RyanRosario. Thanks for your PR.

I'm waiting for a github.com 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.

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