Pass metadata from the front end to LangFuse (and possibly back)

It would be useful to store more metadata along the traces, like UI version, filterschema version/hash... Also some trace id could be pass back to frontend, for further UI integrations like thumbs up/down.

Basically:

• store any metadata sent from front-end to langfuse trace metadata
• return in metadata traceID, traceURL and webhooks to add score, comments, etc.