[docs] Update v1 metrics design doc

[markmc]

Fixes #27298

With this caveat:

design docs like these are naturally a snapshot in time of a design decision, but more discoverable than a random Google doc. It's really hard to be disciplined enough to keep a doc like this up to date

Co-authored by @simon-mo in #26311 and @atalhens in #27301

[mergify]

Documentation preview: https://vllm--27332.org.readthedocs.build/en/27332/

[gemini-code-assist]

Code Review

This pull request provides a valuable update to the metrics design document, bringing it in line with the V1 engine's capabilities. The changes are well-executed, replacing outdated v0 information with a clear and descriptive list of V1 metrics. The restructuring of the document and the consistent use of historical terminology improve readability and context. The move to shorthand links for PRs and issues is also a nice touch for maintainability. Overall, these changes make the documentation more accurate and useful for developers and users working with vLLM's metrics.

[DarkLight1337]

Thanks for updating