Store trace context in emit_event, and link it in 'await_event'

[Aaron1011]

This should result in Sentry displaying a link from the task execution trace (where 'await_event' is called) back to the trace that performed the 'emit_event' call

This is stored in a new 'metadata' json column in the events table, which we can use to add additional per-event information in the future (without needing to wrap the user-provided payload)

---

Note

Adds first-class event metadata for system data (e.g., tracing) and wires it through emit/await paths.

• Schema: Event tables now include metadata jsonb; ensure_queue_tables creates it with comments
• Migration: Alters all existing e_* tables, drops/recreates emit_event (now (queue, name, payload, metadata)) and await_event (now returns (should_suspend, payload, metadata))
• SQL logic: await_event fetches and returns metadata; checkpoint returns omit metadata; emit_event upserts metadata along with payload
• Rust client/context: emit_event_with injects trace context into event metadata (when telemetry enabled); await_event/join receive metadata and link traces; AwaitEventResult updated
• Errors/tests: New TaskError::EmitEventFailed; tests and queries updated to new function signatures

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Trace linking fails when task wakes from suspension

High Severity

The trace context metadata is never linked when a task resumes after being woken by emit_event. When emit_event wakes a sleeping task, it creates a checkpoint containing only the payload (not the metadata). On resume, TaskContext::create loads this checkpoint into checkpoint_cache. When await_event is called, it hits the cache at line 353 and returns directly, bypassing the SQL query that would fetch metadata from the events table. The trace linking code at lines 392-395 is never reached, breaking the PR's stated goal of linking traces from await_event to emit_event.

Additional Locations (1)

• sql/schema.sql#L1496-L1506