Specify attributes for gen_ai external storage references

[aabmass]

Area(s)

area:gen-ai

What's missing?

Follow on to #2179

#2179 recommends uploading references to external storage (like s3 or gcs) and annotating the spans/events with a reference. However, it is not normative about where to put the references.

Describe the solution you'd like

Specify actual attributes for holding a reference to some external storage for

• gen_ai.system_instructions
• gen_ai.input.messages
• gen_ai.output.messages

and what they should contain, e.g. s3://foo/bar/content.json.

092db44 (was removed from #2179) is a good starting point.

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