Sentry Metrics for Rust #938

https://develop.sentry.dev/sdk/telemetry/metrics/

Add support for Sentry Metrics to the Rust SDK!

This will enable users to record counters, gauges, and distributions.

1. Define the Metrics Protocol and trace_metric envelope type in the SDK.
2. Add the Public API (SDK options and methods) as per the docs.
3. Make sure the the SDK follow the documented behavior.
4. Attach default attributes and optional user attributes to the SDK as per docs.
5. Follow requirements for rate limiting, buffering, batching & flushing and build in measures to prevent excessive memory usage.

For general feedback & questions about Sentry Metrics use this GH discussion; for Rust SDK specific feedback & questions use the comment functionality on this GH issue.