Check and log when mandatory event fields are missing or empty

Add check for the case when configuration and/or Prometheus alert content would lead to empty values for mandatory Canopsis event attributes.

• A mandatory event field is missing from config

• A mandatory event field should be filled by a "value" (dynamic content taken from Prometheus alert) but the value is not found or empty in alert

  Examples:

  • value leads to a nonexistent field (a label or annotation that does not exist in some alerts)
  • value leads to existing field but field is set to null or an empty string

Issue a warning when the case is met, such as:

WARNING - No value for field "connector_name": value "labels.somefield" not found or empty in source alert.

In logging level debug, the entire incoming alert should be written to log (if it's not already done).