Add Metrics for dead letter queue

Add an initial set of metrics for the dead letter queue.

Metrics are supplied under the pipeline in the following format:

"dead_letter_queue": {
        "queue_size_in_bytes": ...,
      }

Metrics are populated via a PeriodicPoller

Also fixed up calculation of currentQueueSize to take account
of version headers, which was previously being skipped.

Additionally, whether the dlq is enabled, and if so, the path
of the dlq is supplied under the pipelines API endpoint

Resolves elastic#7287

qa/integration/specs/monitoring_api_spec.rb

@@ -48,25 +48,6 @@
     end
   end
 
-  it 'can retrieve dlq stats' do
-    logstash_service = @fixture.get_service("logstash")
-    logstash_service.start_with_stdin
-    logstash_service.wait_for_logstash
-    Stud.try(max_retry.times, [StandardError, RSpec::Expectations::ExpectationNotMetError]) do
-      # node_stats can fail if the stats subsystem isn't ready
-      result = logstash_service.monitoring_api.node_stats rescue nil
-      expect(result).not_to be_nil
-      # we use fetch here since we want failed fetches to raise an exception
-      # and trigger the retry block
-      queue_stats = result.fetch('pipelines').fetch('main')['dead_letter_queue']
-      if logstash_service.settings.get("dead_letter_queue.enable")
-        expect(queue_stats['queue_size_in_bytes']).not_to be_nil
-      else
-        expect(queue_stats).to be nil
-      end
-    end
-  end
-
   it "can retrieve queue stats" do
     logstash_service = @fixture.get_service("logstash")
     logstash_service.start_with_stdin
@@ -116,7 +97,7 @@
       logging_put_assert logstash_service.monitoring_api.logging_put({"logger." => "INFO"})
       logging_get_assert logstash_service, "INFO", "TRACE"
 
-      #package logger 
+      #package logger
       logging_put_assert logstash_service.monitoring_api.logging_put({"logger.logstash.agent" => "DEBUG"})
       expect(logstash_service.monitoring_api.logging_get["loggers"]["logstash.agent"]).to eq ("DEBUG")
       logging_put_assert logstash_service.monitoring_api.logging_put({"logger.logstash.agent" => "INFO"})