fix(spans): Enable better_backpressure in span buffer RunTask

[untitaker]

Bumps arroyo to 2.39.0 and enables the new better_backpressure=True flag in the span buffer's RunTask.

When the flusher applies backpressure, the current RunTask implementation re-raises MessageRejected and re-runs the user function on each retry attempt. This causes increased CPU load on Redis because add-buffer.lua executes multiple times per message.

With better_backpressure=True, the function runs exactly once per message — transformed messages are stored on backpressure and retried in poll() instead.

See getsentry/arroyo#536 for the arroyo change.

Ref STREAM-881

[linear-code]

STREAM-881 Investigate duplicate spans appearing after buffered-segments

[sentry]

Bug: The better_backpressure=True parameter is missing in a fallback path used only in contrived test scenarios where multiprocessing is disabled but multiple processes are requested.
_{Severity: LOW}

Suggested Fix

In src/sentry/utils/arroyo.py, update the run_task_with_multiprocessing function to pass better_backpressure=True to the RunTask constructor within the fallback logic. This will ensure consistent behavior across all code paths, even those not used in production.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/spans/consumers/process/factory.py#L125

Potential issue: In a specific, non-production scenario, the `better_backpressure=True`
parameter is not applied to the stream processor. This occurs when the
`KAFKA_CONSUMER_FORCE_DISABLE_MULTIPROCESSING` setting is enabled (typically only for
tests) while simultaneously requesting more than one process (`num_processes > 1`). The
primary production path, which uses a single process, correctly receives the
`better_backpressure=True` parameter. The fallback path, handled by
`run_task_with_multiprocessing()` in `src/sentry/utils/arroyo.py`, was not updated to
include this parameter, leading to the inconsistency. However, this combination of
settings is not used in production or existing tests.

Also affects:

• src/sentry/utils/arroyo.py

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[untitaker]

it's legitimate but not worth fixing. i just want to see this rolled out in prod