`gen_ai.response.finish_reasons` not normalized as array

[AlexRuiz7]

Description of the issue:

The field gen_ai.response.finish_reasons is described as an array of values, according to its description and the example provided, however, it is not set as such under the normalization block.

Any additional context or examples:

OpenTelemetry correctly defines it as an array of strings. https://opentelemetry.io/docs/specs/semconv/registry/attributes/gen-ai/#gen-ai-response-finish-reasons

[trisch-me]

@susan-shu-c could you check and update this as you are working on gen-ai fields

[AlexRuiz7]

Related #2560