FastStream keeps its own stderr handler next to lite-bootstrap's, so every FastStream log line is written twice #336

FastStream installs a StreamHandler(sys.stderr) with its colourized text format on the faststream logger at import time (faststream/_internal/logger/logging.py:94-99 in faststream 0.7.4). FastStreamLoggingInstrument.bootstrap (lite_bootstrap/bootstrappers/faststream_bootstrapper.py:155-163) hands brokers structlog.get_logger("faststream"), and MemoryLoggerFactory adds its MemoryHandler to the same stdlib logger on first use (lite_bootstrap/instruments/logging_factory.py:120-133) without removing FastStream's. Every FastStream record is then written as lite-bootstrap JSON on stdout and again, in FastStream's text format, on stderr.

At the default faststream_log_level=logging.WARNING the happy path logs nothing, so this shows up on errors and for services that lower the level. Measured on 2026-10-24 at 705d8a3 on Linux with 2 CPUs (CPython 3.14.9, a real RedisBroker, the subscriber's process_message on a prebuilt message), with faststream_log_level=logging.INFO: removing FastStream's handler as a prototype takes the cost from +35.2 to +25.1 µs per message, 10 µs per message or 5 µs per record ("Received" and "Processed"). The tables and scripts are to be published with #335.

Workaround today: logging_unset_handlers=["faststream"].

Work

• Test first: after bootstrapping a FastStreamBootstrapper with a broker, logging.getLogger("faststream").handlers holds no StreamHandler writing to sys.stderr, and one FastStream INFO record at faststream_log_level=logging.INFO produces one line. It fails today.
• In FastStreamLoggingInstrument.bootstrap, remove the handlers already on the faststream stdlib logger before lite-bootstrap's is added, and put them back in teardown() so a torn-down process logs as it did before bootstrap.
• Alternative to weigh: clear existing handlers in MemoryLoggerFactory.__call__ for every logger it creates. That covers other libraries with the same pattern, but it touches loggers lite-bootstrap does not own, so the FastStream-only fix is the safer default.
• Docs: none; docs/integrations/faststream.md does not describe FastStream's own output.

Verify: the new test passes, and the existing FastStream logging tests in tests/test_faststream_bootstrap.py still pass.

Not breaking: the duplicate stderr line goes away.