Add `first_time_to_token` attribute to `LlmChatCompletionSummary`

[amychisholm03]

Description

“Time To First token” is an important and required metric to measure the model performance. We need to add this metric to AIM via LlmChatCompletionSummary.first_time_to_token.

• This metric should only be captured for streaming requests
• The metric name should be captured as LlmChatCompletionSummary.time_to_first_token
• The metric value should be stored as the number of milliseconds between the time when the request was issued and the first token is received

Pending Questions to Clarify

• Is this for all LLMs or just a select few?
• What is the default/error value? 0, undefined?

Acceptance Criteria

• first_time_to_token is added to the LlmChatCompletionSummary class via this['response.first_time_to_token']
• this attribute can be seen in the UI
• Appropriate tests are created

Additional context

Jira link

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-499484