Add public trace metrics macros and expose the metrics feature

[szokeasaurusrex]

This issue tracks the public API portion extracted from #997: expose metric emission through Rust macros and make the feature available from the top-level crate.

Scope from #997

• sentry-core/src/metrics.rs (metric_count!, metric_gauge!, metric_distribution!)
• sentry-core/src/lib.rs (macro module updates)
• sentry/Cargo.toml (public metrics feature)
• CHANGELOG.md (feature entry)

Details

The macro API is the intended public surface for now, with support for required name/value plus optional attributes and optional unit, matching the metric operation semantics in the trace metrics spec while routing through the existing hub/scope capture path. Tests/examples should demonstrate that emitted metrics are correctly typed and reach batching/capture as expected.

Notes

We are intentionally keeping the macro-based API in this split; if spec wording needs to be adjusted for Rust API shape, that follow-up should happen separately rather than blocking this extraction.

[linear]

RUST-162