v0.4.0

This update has many changes that are intended to be backwards compatible. The
minor version bump is made out of an abundance of caution due to the alarm
description change noted below.

• New features

  • Support registering callback functions, called remedies, to alarm IDs to fix
    the issue that caused the alarm to be set. Managed alarms support automatic
    registration of remedies. This feature doesn't add anything that couldn't
    have been done before, but it reduces boilerplate and some subtle error
    handling code.

  • Add unknown_as_set/1 function for use in alarm_if expressions to assume
    an alarm is set if it hasn't been set or cleared yet. This is useful since
    unknown alarms are assumed to be cleared everywhere else.

  • Add Alarmist.alarm_state/1 for getting the state of a single alarm. This
    function can return :unknown if Alarmist doesn't know anything about the
    alarm.

  • Add Alarmist.Event.timestamp_to_utc/2 helper function for converting the
    monotonic timestamps in event messages to UTC.

  • Add on_time/3 function for use in alarm_if expressions. This function
    tracks the cumulative time that an alarm has been set in an interval. If
    that exceeds a threshold, it returns that a set status.

  • Add sustain_window/3 function for use in alarm_if expressions. This
    function tracks the longest continuous interval that an alarm has been set
    within an interval. If it is above a threshold, then it returns a set status.

• Changes

  • Always send alarm events when managed alarms are added or removed. On
    removal, the alarm transitions to the :unknown state. Previously unknown
    and clear were considered equivalent so they did not trigger an event.

  • For internally generated :set events, always set the description to nil.
    Previously, some sets had empty list descriptions.

  • Change Alarmist.info/1 to only show set alarms by default. Cleared alarms
    can be shown as well via an option, but they were removed since they could
    make the set alarms scroll off the terminal in production systems.

• Fixes

  • Fix exception that's raised when an Alarmist call times out. Thanks to
    @jjcarstens for this fix.