doc: Add metrics documentation and add a 'Reference' section

[Hugoch]

What does this PR do?

Adds metrics description in the /metrics endpoint and in the documentation.

Helps https://github.com/huggingface/text-generation-inference/issues/2227

Before submitting

• [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [X] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR.

@Narsil

[HuggingFaceDocBuilderDev]

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