fix(tracing): do not raise exception if partial flush is triggered without any spans [backport 2.9]

[github-actions]

Backport fffab01 from #9349 to 2.9.

Adds a guard against on_span_finish() with partial flushing on running into an IndexError because there are no spans to flush (which may happen if tracer.configure() was called between the time a span was created and the time it was finished).

In practice, this turns into:

>>> import ddtrace
>>> with ddtrace.tracer.trace("regression"):
...     ddtrace.tracer.configure(partial_flush_min_spans=1)
...
Partial flush triggered but no spans to flush (was tracer reconfigured?)

This also refactors the test for our os.fork() wrapper to have the child process unpatch coverage (just in case, since it occasionally causes exceptions on exit) and exit cleanly (otherwise it would continue running other tests which is not what we want).

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-9349-to-2.9
Commit report: 40a6360
Test service: dd-trace-py

✅ 0 Failed, 116363 Passed, 55167 Skipped, 3h 1m 41.52s Total duration (5h 39m 44.18s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-06-28 08:17:25

Comparing candidate commit eed64d8 in PR branch backport-9349-to-2.9 with baseline commit cf72ae9 in branch 2.9.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 201 metrics, 9 unstable metrics.