Updated for OnApp 5.2 (7th December 2016)
Go to the vCD instance that you have added within OnApp, Settings > Compute Resources > 'Name of vCD instance'.
Click Tools and then 'vCenter Passwords'.
Ensure that all of the fields are correct, in order to connect to all of your vCenter's within your entire vCloud Director environment.
Once the green light is shown for each vCenter, performance metrics will be shown for all virtual machines.