Set system message as separate attribute

[nicohrubec]

So far we put all the messages including the system prompt on gen_ai.request.messages. Instead, the system prompt should be pulled out and set as a separate attribute called gen_ai.system_instructions. For now we will search for the first system message and use that.

The attribute should adhere to the structure given in the otel spec: https://opentelemetry.io/docs/specs/semconv/registry/attributes/gen-ai/#gen-ai-system-instructions

[linear]

JS-1522

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #18978, which was included in the 10.37.0 release.