IGNITE-14261 Add documentation references to the new metrics system (#8844)

Author: Mmuzaf

docs/_docs/monitoring-metrics/metrics.adoc

@@ -18,6 +18,9 @@
 
 == Overview
 
+The Apache Ignite 2.8 release introduced a new mechanism for collecting metrics, which is intended to replace all
+the legacy metrics below. Please, check the link:monitoring-metrics/new-metrics-system.adoc[New Metrics System].
+
 Ignite exposes a large number of metrics useful for monitoring your cluster or application.
 You can use JMX and a monitoring tool, such as JConsole to access these metrics via JMX.
 You can also access them programmatically.

docs/_docs/monitoring-metrics/new-metrics-system.adoc

@@ -18,8 +18,6 @@
 
 == Overview
 
-WARNING: Experimental
-
 Ignite 2.8 introduced a new mechanism for collecting metrics, which is intended to replace the link:monitoring-metrics/metrics[legacy metrics system].
 This section explains the new system and how you can use it to monitor your cluster.
 //the types of metrics and how to export them, but first let's explore the basic concepts of the new metrics mechanism in Ignite.