Error while running action plugin 'alerta_prometheus': name 'USE_AM_EXTERNALURL_FOR_SILENCES' is not defined (500)

[richardsavage]

Issue Summary
When using the prometheus plugin, I get an error, as mentionned in the title:
Error while running action plugin 'alerta_prometheus': name 'USE_AM_EXTERNALURL_FOR_SILENCES' is not defined (500)

The problem seem to be this:

the variable set is:
ALERTMANAGER_USE_EXTERNALURL_FOR_SILENCES = os.environ.get('ALERTMANAGER_USE_EXTERNALURL_FOR_SILENCES') or app.config.get('ALERTMANAGER_USE_EXTERNALURL_FOR_SILENCES',False)

but the plugin is looking for this:

        # if alertmanager is clustered behind a load balancer that mirrors requests we should prefer to create one silence
        # rather than many
        if USE_AM_EXTERNALURL_FOR_SILENCES:

I tried to change the condition to ALERTMANAGER_USE_EXTERNALURL_FOR_SILENCES and it seem to work fine so far

Environment

• OS: LLinux

• API version: 8.6.5

• Deployment: self-hosted

• For self-hosted, WSGI environment: nginx/uwsgi

• Database: MongoDB

• Server config:
  Auth enabled? Yes
  Auth provider? Basic
  Customer views? No
  (or provide the /config endpoint output)

• web UI version: 6.7.0

• CLI version: na

To Reproduce
Steps to reproduce the behavior:

1. Go to the web ui
2. Try to ackonowledge an Alertmanager alert.
   '
3. See error (include command output)

Traceback (most recent call last):
File "/opt/alerta/lib64/python3.6/site-packages/flask/app.py", line 1516, in full_dispatch_request
rv = self.dispatch_request()
File "/opt/alerta/lib64/python3.6/site-packages/flask/app.py", line 1502, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/opt/alerta/lib64/python3.6/site-packages/flask_cors/decorator.py", line 128, in wrapped_function
resp = make_response(f(*args, **kwargs))
File "/opt/alerta/lib64/python3.6/site-packages/alerta/auth/decorators.py", line 90, in wrapped
return f(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/models/metrics.py", line 258, in wrapped
response = f(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/utils/response.py", line 18, in decorated
return func(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/views/alerts.py", line 173, in action_alert
raise ApiError(str(e), 500)

Traceback (most recent call last):
File "/opt/alerta/lib64/python3.6/site-packages/flask/app.py", line 1516, in full_dispatch_request
rv = self.dispatch_request()
File "/opt/alerta/lib64/python3.6/site-packages/flask/app.py", line 1502, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/opt/alerta/lib64/python3.6/site-packages/flask_cors/decorator.py", line 128, in wrapped_function
resp = make_response(f(*args, **kwargs))
File "/opt/alerta/lib64/python3.6/site-packages/alerta/auth/decorators.py", line 90, in wrapped
return f(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/models/metrics.py", line 258, in wrapped
response = f(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/utils/response.py", line 18, in decorated
return func(*args, **kwargs)
File "/opt/alerta/lib64/python3.6/site-packages/alerta/views/alerts.py", line 173, in action_alert
raise ApiError(str(e), 500)

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Expected behavior
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[micwiel]

Bug seems to be fixed in #380 but the fix is still unavailable in alerta docker image (currently latest=8.7.0) https://github.com/alerta/docker-alerta/releases. Please release new image version.