How to create a plugin that have a callback for alert create

[skather2811]

Issue Summary
Need to create a plugin which adds an attribute to alert received from heartbeat . Main goal is blackout using that attribute instead of creating blackout for everyday . That plugin will add this attribute value true/false as per requirement

Environment

• OS: Centos

• For self-hosted, WSGI environment: uwsgi

• Database: MongoDB

• Server config:
  Auth enabled? No

• Created a env using miniconda3

• Located the plugins folder

• create a sample file say holidaybalckout.py there

  `import logging
  from alerta.plugins import PluginBase
  LOG = logging.getLogger('alerta.plugins')
  class HolidayBlackoutReceiver(PluginBase):
  def pre_receive(self, alert, **kwargs):

    LOG.info('Enhancing alert...')
  
    day_of_week = alert.create_time.strftime('%a')
    hour_of_day = alert.create_time.hour
    if day_of_week in ['Sat', 'Sun'] or 8 > hour_of_day > 18:
        alert.attributes['isOutOfHours'] = True
    else:
        alert.attributes['isOutOfHours'] = False
    return alert
  

  def post_receive(self, alert, **kwargs):
  return

  def status_change(self, alert, status, text, **kwargs):
  return

  def take_action(self, alert, action, text, **kwargs):
  raise NotImplementedError

  def delete(self, alert, **kwargs) -> bool:
  raise NotImplementedError`

• Added this file name in configuration file and then restarted uwsgi

Expecting to have an attribute in alert either true/false but nothing was there
Could you please help me with that and how can we have regex for blackout or can we define blackouts on based of attributes ?