Implement trace metric capture functionality in sentry-core

[szokeasaurusrex]

This is the core capture pipeline part from #997: once protocol support exists, implement metric capture through scope/client/options and batching.

Scope from #997

• sentry-core/Cargo.toml (add metrics feature)
• sentry-core/src/clientoptions.rs (enable_metrics, before_send_metric)
• sentry-core/src/client.rs (metric preparation, batcher integration, capture path)
• sentry-core/src/hub.rs (Hub::capture_metric)
• sentry-core/src/scope/real.rs and sentry-core/src/scope/noop.rs (apply_to_metric)
• sentry-core/src/lib.rs (metrics cfg/module updates needed for core behavior)

Details

Keep this focused on the SDK processing portions of the trace metrics spec: disabled fast-path, default metric attributes, scope-derived trace/span association, optional before_send_metric, and enqueue/flush via the shared batcher. This part should also preserve the current behavior that user attributes are only attached when send_default_pii is enabled. Include/retain tests for disabled metrics, batching + flush, and callback filtering/modification.

[linear]

RUST-160