document measurement protocol parameters

Author: evanSegment

src/connections/destinations/catalog/google-analytics/index.md

@@ -621,6 +621,22 @@ If you want to send UTM parameters to Google Analytics using one of the Segment
 
 Your UTM params need to be passed in the `context` object in `context.campaign`. For Google Analytics `campaign.name`, `campaign.source` and `campaign.medium` all need to be sent together for things to show up in reports. The other two params (`campaign.term` and `campaign.content`) are both optional, but will be forwarded to GA if you send them to Segment.
 
+### Measurement Protocol Parameters
+
+Google Analytics uses a reserved set of [Measurement Protocol Parameters](https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters) which are automatically collected by the device-mode Google Analytics tracker. 
+
+To include Measurement Protocol Parameters when sending server-side events to Google Analytics, you must:
+
+1. Collect the value of the parameter as a Segment trait or property
+2. Configure the Segment Google Analytics destination to [map the trait or property](#map-traits-or-properties-to-measurement-protocol-params) to a specific Measurement Protocol Parameter key
+
+The following Measurement Protocol Parameters are currently supported:
+
+- plt
+- pdt
+- gclid
+- dt
+- cid
 
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