Add span events to track streaming request milestones for detailed streaming performance analysis

[psschwei]

Description:
Add span events to track streaming request milestones for detailed streaming performance analysis.

Detailed Requirements:

1. Add span events during streaming:
   • first_token - Time to first token received
   • chunk_received - Periodic chunk delivery (configurable sampling)
   • stream_complete - Stream completion with totals
2. Event attributes:
   • ttfb_ms - Milliseconds to first token
   • chunk_size - Size of chunk in tokens/bytes
   • chunks_received - Total chunks at completion
   • total_tokens - Total tokens at completion
3. Integrate with streaming backends:
   • OpenAI streaming responses
   • Other streaming-capable backends
4. Make chunk event sampling configurable (avoid event spam)

Files to Modify:

• mellea/telemetry/__init__.py - Add span event helpers
• mellea/backends/openai.py - Add streaming events
• Other streaming backends as applicable

Environment Variables:

Variable	Default	Description
MELLEA_TRACE_STREAM_CHUNKS	false	Emit events for each chunk
MELLEA_TRACE_STREAM_SAMPLE_RATE	10	Sample every Nth chunk

Event Schema:

span.add_event("first_token", {"ttfb_ms": 245})
span.add_event("chunk_received", {"chunk_index": 5, "tokens": 12})
span.add_event("stream_complete", {"total_chunks": 15, "total_tokens": 150})

Acceptance Criteria:

• First token event with TTFB timing
• Optional chunk events with sampling
• Completion event with totals
• Works with OpenAI streaming

[ajbozarth]

Closing as part of the tracing epic (#444) re-plan. This sub-issue predates the plugin/hooks pattern (PR #653) and the GenerationMetadata partition (PR #908); its spec has drifted.

@planetf1 is actively building the streaming-events infrastructure this would hook into (#902 / PR #958). The refreshed plan has the tracing-side consumer tracked separately so the two efforts can coordinate cleanly.

Replacement: #1051 — streaming span events, coordinating with #902 / PR #958.

See #444 for the full refreshed plan.