Preliminary Concepts
The 'Abstain' is a configurable option of the Herzum Approval add-on.
It's possible to configure it in the Global Configuration administration screen.
Solution
The customer requirement has been satisfied through the configuration of a Abstain Approval Action from Other Customization section.
Starting from the issue view, in the Approval panel it will appears the bar status, which will show the percentage of approval actions of abstain (blue label)
and the button which will allow the action of abstention (blue button).
If an user abstains the panel will be showed as following.
