Add Langfuse traces

[hanna-paasivirta]

Short Description

Adds Langfuse observability to all three chat services (global_chat, job_chat, workflow_chat) with per-user opt-in control.

Fixes #410

Requested Lightning changes: OpenFn/lightning#4595
We need to figure out self-hosting: #439

Implementation Details

Tracing

Every chat request now produces a structured trace in Langfuse — the full call tree from the top-level main() through routing, planning, subagent calls, RAG retrieval, and Anthropic API calls. Multi-turn conversations are grouped into Langfuse sessions via session_id. Anthropic SDK calls are auto-instrumented via OpenTelemetry.

Tracking controls

• Tracing is opt-in per request via a metrics_opt_in boolean in the payload. When absent or false, a should_export_span filter drops all spans before they leave the process — zero data sent to Langfuse.
• Exception: global_chat is temporarily force-tracked regardless of metrics_opt_in (one-line removal when we're ready to rely on the flag).
• When job_chat/workflow_chat are called as subagents from global_chat, they inherit the parent's tracing context automatically — no need to pass metrics_opt_in through subagent payloads.

User identity

A new user object in the payload carries id (mapped to Langfuse user_id) and employee (added as a Langfuse tag). This lets us filter traces by user and segment employee vs external usage.

API key safety

The @observe decorator on main() was capturing the entire input dict — including api_key — as the trace input. Fixed by adding capture_input=False and explicitly setting only the user's message as the span input.

Payload changes (frontend action needed)

Three new optional fields on all services:

{
  "user": { "id": "user-abc", "employee": true },
  "metrics_opt_in": true,
  "meta": { "session_id": "..." }
}

meta.session_id is new and consistent across all three services (global_chat previously used metadata — renamed to meta for consistency).

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[josephjclark]

work out how to remove API keys from what we send to langfuse. Any other sensitive stuff?

[josephjclark]

next steps here:

• fix the api key thing
• Use an env var to drive which server to talk to
• Only send to langfuse if there's some kind of metrics_opt_in
• (but maybe hard-code the global chat to use langfuse for now?)