Monitor IMAP quota

Promoting :-)

Monitoring sensors are plugins + configuration with full freedom to the user in how to use the sensors. There is no reason to add a submodule - no other module provides monitoring through a submodule.

The patch above accidentally contains parts of the "Connect" button issue.

We usually use drop downs instead of radios.

Also, quota is more a relative thing. So the value should be percent of total.
The message can contain the effective value, but shorter please. Use "addMessage", not "setMessage".

Thus, we can use threshold exceeds for >80% usage.
Plz check how to properly register the percent value... Hope monitoring supports it. :-)

1. +++ b/src/Plugin/monitoring/SensorPlugin/IMAPQuotaSensorPlugin.php
   @@ -0,0 +1,104 @@
   +    foreach ($deliverers as $deliverer) {
   +      // @todo: Replace with getPluginInstance() after https://www.drupal.org/node/2769617.
   +      $plugin = $deliverer_manager->createInstance($deliverer->getPluginId(), $deliverer->getConfiguration());
   

   Yeah? :-)

2. +++ b/src/Plugin/monitoring/SensorPlugin/IMAPQuotaSensorPlugin.php
   @@ -0,0 +1,104 @@
   +      if ($plugin instanceof ImapFetcher) {
   

   Pity there is no method that provides us plugins of a specific type..?

3. +++ b/src/Plugin/monitoring/SensorPlugin/IMAPQuotaSensorPlugin.php
   @@ -0,0 +1,104 @@
   +  public function runSensor(SensorResultInterface $result) {
   

   This needs to survive a situation when the IMAP plugin is not available on server.

4. +++ b/src/Plugin/monitoring/SensorPlugin/IMAPQuotaSensorPlugin.php
   @@ -0,0 +1,104 @@
   +    $imap_fetcher = DelivererConfig::load($this->sensorConfig->settings['imap_fetcher']);
   

   A fetcher could be deleted (or none selected), thus you need to care about failed loads.

5. +++ b/src/Plugin/monitoring/SensorPlugin/IMAPQuotaSensorPlugin.php
   @@ -0,0 +1,104 @@
   +      if ($usage >= $limit) {
   +        $result->setStatus(SensorResultInterface::STATUS_CRITICAL);
   +      }
   +      else {
   +        $result->setStatus(SensorResultInterface::STATUS_OK);
   +      }
   +      $result->setValue($percentage);
   

   A usage>limit is technically not possible.

   You simply setValue.

   A user can setup a threshold >90 to critical in monitoring and things are perfect and much more flexible.

OK committing this.

Did some cleanups... Tests are completely missing, added a @todo, i guess we will revisit this sensor when in use and will then check follow-ups.