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.
From Starting from the issue view it will appears , in the Approval panel it will appears the bar status, which showed will show the percentage of approval actions of abstain (blue label)
and the button which it will allow to abstain the action of abstention (blue button).
If an user abstains the panel will be showed as following.
