OpenAI: Attach custom attributes to llm_events when prefixed with llm.

[hannahramadan]

Customers should be able to add custom attributes to a transaction via add_custom_attribute API. If a custom attribute is prefixed with llm., we should automatically attach it to the llm event, ie llm.<user_defined_metadata>

The conversation_id is now being handled like any other llm.* custom event and should be added to all events when available. Update the references to conversation_id to use this new llm. style.

Let's make sure the attributes are applied to all custom events. Not just chat completion summaries.

We should make sure this is captured in documentation and may need to additionally describe this functionality in the reserved keyword docs.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-227795

[kaylareopelle]

While we're working on this, let's make sure the attributes are applied to all custom events. Not just chat completion summaries.

The conversation_id is now being handled like any other llm.* custom event and should be added to all events when available.

[kaylareopelle]

@hannahramadan - When you get back to this, let me know if you think removing the plumbing for conversation_id is an appropriate add for this ticket. Also happy to break it out into a different one.

[hannahramadan]

@kaylareopelle Yes let's throw that on here ◡̈

[hannahramadan]

@kaylareopelle jsky Im going to remove conversation_id in a different PR. It'll touch a lot of files and I dont want that to detact from this addition!