API access to the span generated by the @logfire.instrument() decorator

[milest]

Description

(as discussed on Slack)

I'd like to have access to the span generated by the decorator. eg of how I'm hoping to use it:

@logfire.instrument()
def lf_example():
    result = do_some_things()
    if result:
        # Set an attribute on the current logfire span from the decorator
        logfire.set_attribute(status='OK')
    else:
        logfire.set_attribute(status='ERROR')

An idea explored on Slack was to use otel apis:

from opentelemetry.trace.propagation import get_current_span
get_current_span().set_attribute(...)

But it seems that the otel apis don't work with non-primatives.

An idea was to add a logfire.get_current_span api that would get the LogfireSpan instance.