Close spans when process shuts down before the exporter shuts down and drops them

[alexmojaki]

Without this PR, this script:

import logfire

def stream():
    with logfire.span('span'):
        yield 1
        yield 2

s = stream()
next(s)

logs an error: "Already shutdown, dropping span."

This PR registers exiting the span context managers with atexit. This happens after the tracer provider is initialized, where OTEL registers its own shutdown (and thus the shutdown of the span processors and exporters) with atexit. Since atexit runs the callbacks in reverse order of registration, that means the spans will be closed by atexit before the exporters shut down.

[codecov]

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All modified and coverable lines are covered by tests ✅

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