feat(sentry-core): Implement trace metric capture batching (#1026)

Basic metrics capture functionality. Follow-up PR will implement the rest.

Closes #1023
Closes [RUST-168](https://linear.app/getsentry/issue/RUST-168/implement-trace-metric-capture-and-batching-in-sentry-core)

Author: szokeasaurusrex

CHANGELOG.md

@@ -19,6 +19,8 @@
 
 - Added a `Envelope::into_items` method, which returns an iterator over owned [`EnvelopeItem`s](https://docs.rs/sentry/0.46.2/sentry/protocol/enum.EnvelopeItem.html) in the [`Envelope`](https://docs.rs/sentry/0.46.2/sentry/struct.Envelope.html) ([#983](https://github.com/getsentry/sentry-rust/pull/983)).
 - Add SDK protocol support for sending trace metric envelope items ([#1022](https://github.com/getsentry/sentry-rust/pull/1022)).
+- Add `TraceMetric` and `TraceMetricType` types representing [trace metrics](https://develop.sentry.dev/sdk/telemetry/metrics/) ([#1026](https://github.com/getsentry/sentry-rust/pull/1026)).
+- Add trace metric capture and batching in `sentry-core`. Metrics can be captured via `Hub::capture_metric` and are batched and sent as `trace_metric` envelope items. Controlled by the `metrics` feature flag and `ClientOptions::enable_metrics` ([#1026](https://github.com/getsentry/sentry-rust/pull/1026)).
 - Expose transport utilities ([#949](https://github.com/getsentry/sentry-rust/pull/949))
 
 ### Fixes