Adding logging to track "No Targets Specified" error fixes VICE-598

Test Plan
- Deploy and observe metric in datadog

Change-Id: Ic86de9b772a72d6fab710d4a58433a843973cae7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/241822
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

app/models/message.rb

@@ -638,9 +638,14 @@ class Message < ActiveRecord::Base
   def enqueue_to_sqs
     targets = notification_targets
     if targets.empty?
+      # Log no_targets_specified error to DataDog
+      InstStatsd::Statsd.increment("message.no_targets_specified",
+                                   short_stat: 'message.no_targets_specified',
+                                   tags: {path_type: path_type})
+
       self.transmission_errors = "No notification targets specified"
       self.set_transmission_error
-    else
+  else
       targets.each do |target|
         Services::NotificationService.process(
           notification_service_id,