feat(prometheus): add postal_message_queue_latency metric

app/lib/message_dequeuer/initial_processor.rb

@@ -3,6 +3,8 @@
 module MessageDequeuer
   class InitialProcessor < Base
 
+    include HasPrometheusMetrics
+
     attr_accessor :send_result
 
     def process
@@ -10,6 +12,7 @@ module MessageDequeuer
         logger.info "starting message unqueue"
         begin
           catch_stops do
+            increment_dequeue_metric
             check_message_exists
             check_message_is_ready
             find_other_messages_for_batch
@@ -28,6 +31,13 @@ module MessageDequeuer
 
     private
 
+    def increment_dequeue_metric
+      time_in_queue = Time.now.to_f - @queued_message.created_at.to_f
+      log "queue latency is #{time_in_queue}s"
+      observe_prometheus_histogram :postal_message_queue_latency,
+                                   time_in_queue
+    end
+
     def check_message_exists
       return if @queued_message.message
 

app/lib/worker/process.rb

@@ -288,6 +288,9 @@ module Worker
       register_prometheus_histogram :postal_worker_task_runtime,
                                     docstring: "The time taken to process tasks",
                                     labels: [:task]
+
+      register_prometheus_histogram :postal_message_queue_latency,
+                                    docstring: "The length of time between a message being queued and being dequeued"
     end
 
   end