feat: Add completion start time timestamp to relevant generators

[vblagoje]

Why:

Enhances Langfuse integration to track the time-to-first-token (TTFT) in streaming responses. The new entry in chat message metadata completion_start_time enables TTFT tracking in Langfuse. We'll separately update all other relevant generators in https://github.com/deepset-ai/haystack-core-integrations/

What:

• Added completion_start_time metadata in the Hugging Face API and OpenAI chat and non-chat generators to record the TTFT.
• Modified test cases to validate the inclusion of the completion_start_time and its format correctness.

How can it be used:

This change allows developers to track the exact start time of streaming completions, aiding in performance diagnostics

How did you test it:

Included updated unit tests for both Hugging Face API and OpenAI chat and non-chat generators to ensure the presence and correct ISO format of completion_start_time metadata. These tests verify that the timestamps are recorded and formatted correctly during streaming responses.

Notes for the reviewer:

Ensure close attention to the new metadata handling and its correctness across different components. Testing has validated the format, but real integration checks are recommended for full assurance.

[coveralls]

Pull Request Test Coverage Report for Build 12810835892

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 13 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.009%) to 91.301%

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Files with Coverage Reduction	New Missed Lines	%
components/generators/hugging_face_api.py	2	96.3%
components/generators/chat/hugging_face_api.py	3	96.46%
components/generators/chat/openai.py	4	96.3%
components/generators/openai.py	4	95.65%

Totals
Change from base Build 12794162077:	0.009%
Covered Lines:	8848
Relevant Lines:	9691

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💛 - Coveralls

[anakin87]

Suggested change

	meta = {
	"finish_reason": finish_reason if finish_reason else None,
	"received_at": datetime.now().isoformat(),
	}
	chunks_meta.append(meta)
	if first_chunk_time is None:
	first_chunk_time = meta["received_at"]
	meta = {}
	if finish_reason:
	meta["finish_reason"] = finish_reason
	if first_chunk_time is None:
	first_chunk_time = datetime.now().isoformat()

Do you see any problems with this simpler approach?

[anakin87]

same as above - could we simplify it?