Add token usage logging and dashboard for inference plugin

## Summary

This adds unified token logging to the inference plugin, and a dashboard for users to see the token logging.

**Why this needs docs:** New feature — documentation needed.

## Resources

- PR [#260956](https://github.com/elastic/kibana/pull/260956) — Add token usage logging and dashboard for inference plugin

## Availability

| Channel | Details |
|---------|---------|
| **Stack** | v9.5.0 |
| **Serverless** | Apr 20–Apr 24 |
| **Feature flag** | None — active by default |

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*Created with [Docs Quest Scanner](https://github.com/florent-leborgne/docs-quest-scanner) by @kosabogi*

Start with PR #260956 and the issue summary to understand the inference plugin's token logging and dashboard feature. Define the documentation scope for Stack v9.5.0 and Serverless, then consider the work done when the new feature is documented for users.

Written by the indexing model from the issue text.