fix: send buffered telemetry on the background worker

[ikraamg]

Description

TelemetryEventBuffer#send_items runs under @mutex, both from the 5 s timer thread and from the add_item call that reaches max_items. It built the envelope and did the HTTP request to Sentry right there. Two effects:

• The application thread that added the 100th log event paid a full TCP + TLS + POST round trip inline. With sentry-rails 7.0 turning structured logging on by default with the ActiveRecord subscriber, that is every ~100 SQL statements per process.
• Every other thread calling add_item (again, every SQL statement) blocked on the mutex behind that send.

This hands the send to Sentry.background_worker, the same path error events and Client#capture_envelope already use. The envelope is still built under the lock; only the network leaves it. The rescue moves inside the posted block so a failed send is logged the way it was, and with background_worker_threads = 0 the behavior is unchanged (immediate executor).

Measured on a Rails API serving ~37 req/s at ~12 queries per request, after upgrading 6.4.1 → 7.0.0 with default settings: the inline flush was 15.8% of request time in an rbspy profile of the Puma workers (Sentry::TelemetryEventBuffer#send_items → Net::HTTP#connect → TCPSocket#initialize), and request p90 stepped from 44 ms to 66 ms in the hour the upgrade deployed while p50 barely moved (28 → 30 ms). Applies to MetricEventBuffer too, since it shares the base class.

Tests: the shared buffer examples get two cases that run a real one-thread worker and assert the envelope is sent off the thread that filled the buffer (both add_item overflow and flush). Full sentry-ruby suite green locally (1520 examples).

🤖 Generated with Claude Code

https://claude.ai/code/session_012QsjWMGPww2cFqGMX71vkB

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