When you work with mailtips, you can see for this command.
The Group metrics is used to support mailtips for a large audience. Technet says more than 25 recipients is considered a large audience. Group metrics data is generated on designated Mailbox Servers.
You can use the parameter ForceGroupMetricsGeneration $True to enable. This parameter belong to Set-Mailbox cmdlet.
Reference:
https://technet.microsoft.com/en-us/library/jj674302(v=exchg.150).aspx
https://technet.microsoft.com/en-us/library/aa998651(v=exchg.160).aspx
