Document Rust trace metrics support and macro API

[szokeasaurusrex]

Once the sentry-rust split from getsentry/sentry-rust#997 lands, update docs to cover Rust trace metrics usage and caveats in a single user-facing guide.

Scope from getsentry/sentry-rust#997

Reference behavior introduced by getsentry/sentry-rust#997: metrics feature flag exposure, metric_count!/metric_gauge!/metric_distribution! macros, optional attributes and unit, and interaction with enable_metrics / before_send_metric.

Details

Document how to enable the feature, emit each metric type, and understand key runtime behavior (batching, filtering via callback, and rate-limit effects), using terminology consistent with the trace metrics spec. Include examples that mirror the shipped Rust API so users can copy/paste quickly.

Notes from @dingsdax

• create user docs (see Python, Javascript)
• clarify whether to add quickstart example for your SDK
• when merged, change SDK from upcoming to supported

[linear]

DOCS-2435