Support legacy gen_ai token usage attributes

[haasonsaas]

Summary

Workshop currently drops token counts from the raindrop-ai core-SDK instrumentation path because the producer and consumer disagree on the gen_ai.* token attribute names.

This change teaches parseOtlpRequest to read the legacy OpenLLMetry names in addition to the newer OTel semantic convention names:

• gen_ai.usage.prompt_tokens
• gen_ai.usage.completion_tokens

The existing Vercel AI SDK and new-semconv paths stay first in the lookup order, so this is additive for paths that already work.

Root cause

raindrop-ai bundles OpenLLMetry instrumentations such as @traceloop/instrumentation-openai@0.19.0-otel-v1 and the matching Anthropic, Bedrock, Cohere, Vertex, Together, etc. instrumentations. Those emit the legacy token attributes:

gen_ai.usage.prompt_tokens     = 1357
gen_ai.usage.completion_tokens = 82

Workshop ingest only read the newer OTel GenAI semantic convention names for gen_ai.* usage, plus Vercel AI SDK's ai.usage.* attributes:

const inputTokens = first(attrs, "ai.usage.inputTokens", "ai.usage.promptTokens", "ai.usage.prompt_tokens", "gen_ai.usage.input_tokens");
const outputTokens = first(attrs, "ai.usage.outputTokens", "ai.usage.completionTokens", "ai.usage.completion_tokens", "gen_ai.usage.output_tokens");

That means spans from the core-SDK instrumentModules path landed with null / missing input_tokens and output_tokens, so Workshop could not show per-call or per-run token/cost data. The AI SDK path was unaffected because it emits ai.usage.*, which Workshop already handled.

Fix

Append the legacy OpenLLMetry keys to the parser fallback lists:

- "gen_ai.usage.input_tokens")
+ "gen_ai.usage.input_tokens", "gen_ai.usage.prompt_tokens")

- "gen_ai.usage.output_tokens")
+ "gen_ai.usage.output_tokens", "gen_ai.usage.completion_tokens")

first() returns the first attribute that exists, so appending these names preserves the current precedence for AI SDK and new-semconv spans.

Validation

• Installed dependencies with Bun 1.3.14 using the existing frozen lockfile.
• bun test tests/ passes locally.
• bun run build passes locally, including migration embed check, tsc --noEmit, and the Vite UI build.
• Added tests/parse.test.ts covering the legacy OpenLLMetry attribute pair.