WIP: metrics macro

[szokeasaurusrex]

• wip
• more wip
• wip cleanup
• wip more simplify
• wip manual edit
• wip cleanup good now
• add nice error

Closes #1010
Closes RUST-162

[github-actions]

	Fails
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## Unreleased

### Features

- metrics macro ([#1067](https://github.com/getsentry/sentry-rust/pull/1067))

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[linear-code]

RUST-162 Add public trace metrics macros and expose the metrics feature