Create docs for the metrics tab (#29414)

Adds information about the new metrics tab in the ray dashboard. Links to instructions on how to set it up.

doc/source/ray-core/ray-dashboard.rst

@@ -7,7 +7,7 @@ Ray users to understand Ray clusters and libraries.
 
 The dashboard lets you:
 
-- View cluster metrics.
+- View cluster metrics including time-series visualizations.
 - See errors and exceptions at a glance.
 - View logs across many machines.
 - See all your ray jobs and the logs for those jobs.
@@ -103,6 +103,18 @@ The log viewer provides various search functionality to help find the log messag
 .. image:: https://raw.githubusercontent.com/ray-project/Images/master/docs/new-dashboard/logs-content.png
     :align: center
 
+Metrics View
+~~~~~~~~~~~~~
+The metrics view lets you view visualizations of the time series metrics emitted by Ray. It requires that prometheus and grafana is running for your cluster.
+Instructions about that can be found :ref:`here <ray-metrics>`.
+
+You can select the time range of the metrics in the top right corner. The graphs refresh automatically every 15 seconds.
+
+There is also a convenient button to open the grafana UI from the dashboard. The Grafana UI provides additional customizability of the charts.
+
+.. image:: https://raw.githubusercontent.com/ray-project/Images/master/docs/new-dashboard/metrics.png
+    :align: center
+
 Advanced Usage
 --------------