(AzureCXP) adds details for custom perf counters

Adds information to specify custom perf counters not available by default. Also includes information to escape '&' character in counter name to avoid issues with agent on the source VM.

Author: AnuragSingh-MSFT

articles/azure-monitor/agents/data-collection-rule-azure-monitor-agent.md

@@ -65,7 +65,12 @@ You can define a data collection rule to send data from multiple machines to mul
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     :::image type="content" source="media/data-collection-rule-azure-monitor-agent/data-collection-rule-data-source-basic-updated.png" lightbox="media/data-collection-rule-azure-monitor-agent/data-collection-rule-data-source-basic-updated.png" alt-text="Screenshot that shows the Azure portal form to select basic performance counters in a data collection rule." border="false":::
 
-1. Select **Custom** to collect logs and performance counters that aren't [currently supported data sources](azure-monitor-agent-overview.md#data-sources-and-destinations) or to [filter events by using XPath queries](#filter-events-using-xpath-queries). You can then specify an [XPath](https://www.w3schools.com/xml/xpath_syntax.asp) to collect any specific values. For an example, see [Sample DCR](data-collection-rule-sample-agent.md).
+1. Select **Custom** to collect logs and performance counters that aren't [currently supported data sources](azure-monitor-agent-overview.md#data-sources-and-destinations) or to [filter events by using XPath queries](#filter-events-using-xpath-queries). You can then specify an [XPath](https://www.w3schools.com/xml/xpath_syntax.asp) to collect any specific values.
+
+    For performance counter not available by default, use the following format to add it for collection - `\PerfObject(ParentInstance/ObjectInstance#InstanceIndex)\Counter`. If the counter name contains ampersand (&), replace it with `&amp;`. For example, `\Memory\Free &amp; Zero Page List Bytes`.
+   
+    For examples of DCR, see [Sample DCR](data-collection-rule-sample-agent.md).
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     :::image type="content" source="media/data-collection-rule-azure-monitor-agent/data-collection-rule-data-source-custom-updated.png" lightbox="media/data-collection-rule-azure-monitor-agent/data-collection-rule-data-source-custom-updated.png" alt-text="Screenshot that shows the Azure portal form to select custom performance counters in a data collection rule." border="false":::