feat(arq): Add cron monitoring for arq cron jobs

[wedamija]

Description

Part of a series adding automatic Crons monitoring for the most-used job schedulers in each SDK. Check-ins for a monitor that doesn't exist yet are dropped, so today each monitor has to be created by hand with its schedule copied from the code. Sending the job's schedule as the monitor config lets Sentry create the monitor and keep it in sync.

This adds opt-in check-ins for arq cron jobs (cron(...) in cron_jobs), modelled on CeleryIntegration(monitor_beat_tasks=True, exclude_beat_tasks=[...]):

sentry_sdk.init(
    integrations=[ArqIntegration(monitor_cron_jobs=True, exclude_cron_jobs=["cleanup"])],
)

• The worker sends in_progress when a cron job starts and ok or error when it ends.
• The slug is the cron job's name, without arq's default cron: prefix (so cron(send_report) becomes send_report). exclude_cron_jobs takes regexes, matched like exclude_beat_tasks.
• The monitor config is a crontab built from the month/day/weekday/hour/minute fields, in the worker's timezone. Fixed UTC offsets (arq's default is the system's offset at startup) are sent as Etc/GMT±N. Before arq 0.25, cron jobs run in local time, so the config is only sent when the system timezone is UTC.
• Check-ins are sent without a config when the schedule can't be represented: a second other than 0, both day and weekday set (arq requires both to match, cron either), or a timezone with no IANA name (such as a UTC+05:30 offset).
• run_at_startup runs are checked in like any other run.

Issues

Docs: getsentry/sentry-docs#20056

[github-actions]

Coverage Results 📊

✅ 133854 passed | ⏭️ 7294 skipped | Total: 141148 | Pass Rate: 94.83% | Execution Time: 442m 55s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +206
Passed Tests	📈 +188
Failed Tests	—
Skipped Tests	📈 +18

All tests are passing successfully.

✅ Patch coverage is 94.81% (73 of 77 changed executable lines covered; target 80%).
Project statement coverage is 90.24% (down 0.01 percentage points from base (0f7acd5) to head (9b8ca99)).

Changed files with executable lines (1)

File	Patch coverage	Changed executable lines
sentry_sdk/integrations/arq.py	94.81%	73/77 covered; missed: 361, 363, 391, 416; partial branches: 360, 362, 390, 409

Coverage diff

@@            Coverage Diff             @@
##        master     #8051       +/-##
==========================================
- Coverage    90.25%    90.24%    -0.01%
==========================================
  Files          202       202         —
  Tracked lines     26660     26736       +76
  Branches      9926      9968       +42
==========================================
+ Hits         24062     24127       +65
- Misses        2598      2609       +11
- Partials      1510      1514        +4

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