Added precation notice when internal monitoring collector is used. Cl…

…oses elastic#11346

x-pack/lib/monitoring/monitoring.rb

@@ -104,6 +104,12 @@ def after_agent(runner)
 
         return unless monitoring_enabled?(runner.settings)
 
+        deprecation_logger.deprecated(
+            "Monitoring LogStash using internal collector is deprecated and be removed in version 8.0.\n"\
+            "Please, configure Metricbeat to monitor LogStash, documentation can be found at: \n"\
+            "https://www.elastic.co/guide/en/logstash/current/monitoring-with-metricbeat.html"
+            )
+
         logger.trace("registering the metrics pipeline")
         LogStash::SETTINGS.set("node.uuid", runner.agent.id)
         internal_pipeline_source = LogStash::Monitoring::InternalPipelineSource.new(setup_metrics_pipeline, runner.agent)