Add traffic size metrics

[OmarElawady]

What does this PR do?

Adds bytes_sent and bytes_received metrics for all metric backends.

Motivation

We make a gateway solution based on traefik, and this metric is needed to know how much traffic was used by each service. And it may be useful for other use cases as well, also it would be great to have your feedback whether this is the correct way to calculate it.

More

• Added/updated tests
• Added/updated documentation

Additional Notes

• The relevant tests passes but the integration tests seems to fail at random unrelated tests each time.
• The pilot backend seems to be missing the router metrics. By accident?

[kmskpraveen]

Any update on this? This is a very crucial metrics that will be helpful especially with load testing.

[rtribotte]

Hello @kmskpraveen,

Thanks for your interest in Traefik and thanks @OmarElawady for your contribution!

This PR is labeled as needs-design-review,
as you can see in our contribution guide, this means that we have to take a longer look to evaluate how it would interact with the other parts of Traefik.

We will come back to you once the design review iteration is done.

[tomMoulard]

Hello @OmarElawady,

Thanks for your PR, I've worked a bit on this topic, and opened a new PR based on your work.

I'm closing this PR in favor of #9208.