[Doc]: Update metrics documentation to remove V0 references and add v1 changes.

[atalhens]

Problem

The metrics documentation in docs/design/metrics.md still contains references to V0 metrics implementation, but V0 metrics have been removed after @njhill 's PR #27215 was merged. To avoid confusion, I think we should remove this and update it with the new set of v1 metrics.

Was curious if we want to keep this v0 reference and add the v1 details on top of this.

Suggest a potential alternative/fix

1. Remove all V0 references from the metrics documentation.
2. Update the introduction to focus on V1 metrics only.

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[markmc]

See also #26311

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