[OTel] Three different AnyValue decoders, two of them lossy

[krisztianfekete]

We decode OTel AnyValue in three places and only one does it right. extraction.py silently drops array/kvlist/bytes values, loader/otlp.py JSON-dumps the raw proto structure, and only otlp_processing.py decodes it properly (and only for log bodies).

Concrete symptom: gen_ai.response.finish_reasons shows up as the literal blob {"values":[{"stringValue":"stop"}]} via one path and vanishes via another. One shared decoder fixes all three call sites.

[Leroyyyyyyyyy]

Hi @krisztianfekete — I'd like to take this one, if it's still available.
Planning to have a PR up (with tests) in the next few days.

[krisztianfekete]

Hey, sounds good, looking forward to it!