Don't start every MetricsCollectorWorker

Fix the `#workers` method for any worker that includes the
PerEmsTypeWorkerMixin (aka just the MetricsCollectorWorkers).

This was setting an instance variable `@workers` but this wasn't
actually being called so the ivar was never set up.  This led to every
metrics collector worker for every EMS type to be started if the role
was enabled.

app/models/mixins/per_ems_type_worker_mixin.rb

@@ -3,13 +3,15 @@ module PerEmsTypeWorkerMixin
 
   included do
     self.check_for_minimal_role = false
-    @workers = lambda do
-      return 0 unless self.any_valid_ems_in_zone?
-      workers_configured_count
-    end
   end
 
   module ClassMethods
+    def workers
+      return 0 unless any_valid_ems_in_zone?
+
+      super
+    end
+
     def ems_class
       parent
     end