DOCSP-44318: Metrics collected at different intervals (#808)

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- [DOCSP-44318](https://jira.mongodb.org/browse/DOCSP-44318)
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(cherry picked from commit 87fc90a)

Author: norareidy

source/reference/config/ui-settings.txt

@@ -371,6 +371,12 @@ Default Monitoring Data Retention
 |onprem| computes the data based on the averages from the previous
 granularity level.
 
+.. note::
+
+   |onprem| gathers :guilabel:`Replication Lag` and :guilabel:`Replication Headroom`
+   metrics at an 85-second granularity regardless of your project's
+   default granularity level.
+
 The table determines the default settings for new groups. If you change
 the settings, |onprem| prompts you whether to also apply the settings
 to existing groups. To change the settings for a specific group without