Add docs for metric explorer alerts (elastic#62314)

* Add docs for metric explorer alerts

* Fix link

* Actually fix the link

* Grammar fix

Co-Authored-By: Brandon Morelli <bmorelli25@gmail.com>

Co-authored-by: Brandon Morelli <bmorelli25@gmail.com>

docs/infrastructure/index.asciidoc

@@ -21,6 +21,8 @@ You can optionally save these views and add them to {kibana-ref}/dashboard.html[
 
 * Seamlessly switch to view the corresponding logs, application traces or uptime information for a component.
 
+* Create alerts based on metric thresholds for one or more components.
+
 To get started, you need to <<xpack-metrics-getting-started, start collecting metrics data from your infrastructure>>. Then you can <<infra-ui, use the Metrics application to inspect and analyze your data>>.
 
 [role="screenshot"]

docs/infrastructure/metrics-explorer.asciidoc

@@ -20,6 +20,7 @@ By default that is set to `@timestamp`.
 * The interval for the X Axis is set to `auto`.
 The bucket size is determined by the time range.
 * To use *Open in Visualize* you need access to the Visualize app.
+* To use *Create alert* you need to {kibana-ref}/alerting-getting-started.html#alerting-setup-prerequisites[set up alerting].
 
 [float]
 [[metrics-explorer-tutorial]]
@@ -67,4 +68,8 @@ Choose a graph, click the *Actions* dropdown and select *Open In Visualize*.
 This opens the graph in {kibana-ref}/TSVB.html[TSVB].
 From here you can save the graph and add it to a dashboard as usual.
 
+9. You can also create an alert based on the metrics in a graph.
+Choose a graph, click the *Actions* dropdown and select *Create alert*.
+This opens the {kibana-ref}/defining-alerts.html[alert flyout] prefilled with mertrics from the chart.
+
 Who's the Metrics Explorer now? You are!