Add `logfire.metric_gauge()`

[Kludex]

• Closes https://github.com/pydantic/logfire/issues/145

Checklist

• [x] Add tests.
• [ ] Add documentation.
  • [ ] Improve the explanation on "why" to use each metric, and give examples.

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All modified and coverable lines are covered by tests :white_check_mark:

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[adriangb]

Thanks I needed this the other day 😆