[Feature/Bug] Generic tags in trace payload are nested in metadata for Langfuse or Langsmith, breaking native UI.

[Aksherwal]

Context:
When using OpenRouter's raw HTTP payload to broadcast observability data to Langfuse, passing an array of "tags" inside the trace object does not populate Langfuse's native Tags UI.

Expected Behavior:
Just as OpenRouter natively maps "ls.tags" for LangSmith, passing "tags": ["prod", "testing"] inside the trace object should result in top-level OpenTelemetry span attributes that Langfuse recognizes and promotes to its native UI tags. (Note: The OpenRouter "Send Test Trace" button achieves this perfectly).

Actual Behavior:
Because OpenRouter's generic OTEL exporter does not recognize the generic "tags" key, it wraps the array inside a generic "metadata" dictionary envelope before broadcasting.
Because the tags are nested in metadata rather than sent as top-level span attributes, Langfuse displays them as raw JSON metadata instead of native UI pills.

Steps to Reproduce:

1. Send a raw HTTP request to /chat/completions.

2. Include the following trace object:

"trace": {
  "trace_id": "valid-uuid-here",
  "tags": ["testing", "observability"]
}

3. View the trace in Langfuse. The tags will appear in the Metadata table, not the Tags column.

The Current Workaround:
Developers must bypass the generic "tags" key and explicitly pass Langfuse's proprietary OTEL namespace key in the payload:

"trace": {
  "langfuse.trace.tags": ["testing", "observability"]
}

Proposed Solution:

API Fix: Update the OpenRouter OTEL translation layer to automatically map a generic "tags" array inside the trace object to standard top-level OTEL tag attributes (or specifically to langfuse.trace.tags when broadcasting to Langfuse).

Docs Fix (Alternative): Explicitly document this behavior on the OpenRouter Observability docs page, noting that users must pass "langfuse.trace.tags" to achieve native UI tags in Langfuse.