[Go] Bring prompt tracing/observability up to parity with TS #4063
Description
To bring Go up to parity, prompts should be represented by a span (named after the prompt), with 2 child spans, 'render' and 'generate'.
- For 'render', the "input" is the prompt input (i.e. variable bindings) and the "output" is the GenerateRequest. i.e. should match the "input" to the "generate" call.
- 'generate' is the same as always, just nested under the prompt
