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

[szokeasaurusrex]

Implement issue #1023 by adding the sentry-core capture path and batch lifecycle for TraceMetric.

• Add metrics feature in sentry-core/Cargo.toml
• Compile batcher for client + (logs|metrics)
• Add ClientOptions::enable_metrics (default: true) and include it in Debug
• Add Hub::capture_metric(TraceMetric)
• Add Client metric capture with metrics_batcher lifecycle handling (clone/flush/close)
• Add Batch impl for TraceMetric in batcher
• Add metrics tests for disabled metrics, batch-size flush (150 -> 2 envelopes), and teardown flush

Closes #1023
Closes RUST-168
Co-authored-by: Joris Bayer joris.bayer@sentry.io

[linear-code]

RUST-168 Implement trace metric capture and batching in sentry-core

[github-actions]

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## Unreleased

### Features

- Implement trace metric capture batching ([#1026](https://github.com/getsentry/sentry-rust/pull/1026))

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