subtract task eta from queue wait so countdowns aren't counted

[tulioz]

queue wait was measured from task-sent, so countdown/ETA dispatch counted the intentional delay as wait. measure from when the task is due (max of sent, eta) instead. same fix for the oldest-queued-age gauge.

[aviator-app]

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[gemini-code-assist]

Code Review

This pull request updates queue wait time and oldest queued task age calculations to account for scheduled tasks (ETA/countdown), measuring wait times from when a task is actually due to run rather than when it was sent. Feedback highlights a critical issue where parsing timezone-naive ISO-8601 strings treats them as local system time instead of UTC, and a fix is suggested to explicitly apply UTC timezone info.

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[gemini-code-assist]

When parsing a timezone-naive ISO-8601 string (which Celery events typically use for UTC timestamps), datetime.datetime.fromisoformat(value).timestamp() treats the naive datetime as being in the local system timezone of the machine running celerymon. If the system timezone is not UTC, this will result in incorrect timestamps for eta_ts and expires_ts, leading to incorrect queue wait times and oldest queued age calculations.

To fix this, we should explicitly treat timezone-naive datetimes as UTC:

def _parse_timestamp(value: Any) -> float | None:
    if value is None:
        return None
    if isinstance(value, (int, float)):
        return float(value)
    if isinstance(value, str):
        try:
            dt = datetime.datetime.fromisoformat(value)
            if dt.tzinfo is None:
                dt = dt.replace(tzinfo=datetime.timezone.utc)
            return dt.timestamp()
        except ValueError:
            return None
    return None