Add trace_metric rate limiting behavior for transport

[szokeasaurusrex]

This is the transport rate-limiting part from #997, extracted so it can be reviewed independently of protocol and core capture changes.

Scope from #997

• sentry/src/transports/ratelimit.rs

Details

Extract support for the trace_metric category in limiter state/header parsing and map metric envelope items to that category when deciding whether an envelope can be sent, aligning with the data-category behavior expected by the trace metrics spec. Tests in this area should continue to verify header parsing and category disable checks, now including trace_metric.

[linear]

RUST-161