fix(metrics): Do not wait for flusher thread to join

[sentrivana]

Under uWSGI the metrics flusher thread doesn't join properly, hanging on acquiring an internal lock. At the point where we try to join we've already stopped the thread's loop by setting _running to False, and we don't explicitly need to wait for the thread to terminate.

This mirrors what we do in the BackgroundWorker, where we also don't explicitly join.

Closes #2699

---

General Notes

Thank you for contributing to sentry-python!

Please add tests to validate your changes, and lint your code using tox -e linters.

Running the test suite on your PR might require maintainer approval. Some tests (AWS Lambda) additionally require a maintainer to add a special label to run and will fail if the label is not present.

For maintainers

Sensitive test suites require maintainer review to ensure that tests do not compromise our secrets. This review must be repeated after any code revisions.

Before running sensitive test suites, please carefully check the PR. Then, apply the Trigger: tests using secrets label. The label will be removed after any code changes to enforce our policy requiring maintainers to review all code revisions before running sensitive tests.

[sentrivana]

Merged as part of #2694