Langfuse integration: completionStartedAt is not logged, required for streaming metrics like TTFT and TPM/TPS

[marcklingen]

Describe the bug

When using the Langfuse integration, time to first token metrics are not available (the report I received mentioned Azure OpenAI but this might apply to all LLMs).

Langfuse supports time to first token via the “completionStartedAt” attribute on generations.

To Reproduce

Use Langfuse integration with AzureOpenAI in Haystack.

Describe your environment (please complete the following information):

Reported by user. Can try to get details on this if helpful.

[marcklingen]

This is a bit vague as it was reported by a user but I assume the attribute is not set at all and thus it could be sufficiently detailed.

I think this would be a great addition to the integration.