Add new_trace parameter to logfire.instrument

[njz-cvm]

As per #1493, it's current a bit awkward to remove context from instrumentation. This PR adds the "new_context" keyword to ensure designated function calls do not inherit from parent spans.

Here is an example from the new test:

def context_factory():
    return {'new_contex': 123}

@tagged.instrument('hello-world {a=}', record_return=True, new_context=context_factory)
def hello_world(a: int) -> str:
    return f'hello {a}'

@tagged.instrument('parent', record_return=True)
def parent() -> str:
    return hello_world(5)

Alternatively, new_context can be set to True, which is the same as new_context=dict.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[alexmojaki]

We didn't discuss this context factory API, I'm not sure if I like it or understand the point. It's certainly not being tested here, new_contex doesn't do anything or appear in the exported spans.

[njz-cvm]

I didn't mention it in the original issue because it wasn't something I'd considered. But when I realized I'd just be using attach_context behind the scenes, it seemed preferable to allow users the option to provide input for attach_context.

If you're against it, it's not that useful for me and I don't mind cutting it. But if it's worth keeping I'll add better testing for it.

[alexmojaki]

this feature isn't really compelling enough without the span link

[njz-cvm]

this feature isn't really compelling enough without the span link

I'll look at adding it, just trying to determine the best approach.

[alexmojaki]

If you'd still like me to review this, please ensure that all checks are passing.

[njz-cvm]

Sorry, I didn't have time to finish the required tests. I have changed the API to be what we previously discussed, with new_context being simply True / False. I agree with you that this makes the most sense.

[Copilot]

Pull request overview

This PR adds a new_trace parameter to the instrument decorator, enabling instrumented functions to start a new trace instead of being a child of the current span. When new_trace=True, the function creates a new trace with a span link to the parent span, effectively breaking the parent-child relationship while maintaining traceability.

Key Changes

• Added new_trace boolean parameter to @instrument() decorator
• New traces include a span link to the previous span context for traceability
• Implementation uses OpenTelemetry's context manipulation to detach from parent spans

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/test_logfire.py	Added comprehensive test cases covering new_trace functionality with parent spans, no parent spans, without argument extraction, and with selective argument extraction
logfire/_internal/main.py	Updated _fast_span, _instrument_span_with_args, and instrument method signatures to accept and propagate new_trace parameter via **kwargs
logfire/_internal/instrument.py	Implemented core new_trace logic using OpenTelemetry trace context management, creating span links and detaching from parent context when enabled

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[alexmojaki]

@njz-cvm does this look good to you?

[njz-cvm]

Looks great to me, this is a much cleaner implementation.