feat: streaming span events

[ajbozarth]

Background

Retired sub-issue #476 proposed span events for streaming. @planetf1's open work on streaming event types (#902 / #1013, PRs #958 and #942) is the right place for this to land — the bridge between Mellea's internal streaming events and OTel span events.

Scope

Pending @planetf1's streaming-events design, add span events on backend generation spans for:

• gen_ai.choice or equivalent first-token event (carries ttfb_ms — currently metrics-only).
• Chunk received events (gated by MELLEA_TRACES_STREAM_CHUNKS per refactor: rename tracing env vars to plural and align with OTel semconv #1046's plural convention, to avoid noise).
• Stream complete event.

Optionally surface MELLEA_TRACES_STREAM_SAMPLE_RATE for chunk sampling.

Phase & dependencies

Phase 2 (coverage). Depends on Phase 1 (#1045, #1046, #1047). Coordinates with #902 / PR #958 — this issue is the consumer side; @planetf1's work is the producer side. Can proceed in parallel with #1048, #1049, #1050, #1052.

Acceptance criteria

• Streaming generations emit span events for TTFB and completion.
• Chunk-level events are opt-in and rate-controllable.
• Coordinated with @planetf1's streaming-events work.

Parent epic: #444