Create a business rule whenever the Incident state changes from On hold to any other state until it gets resolved.
I have referred metric_instance table to get the start and end time of on hold state.
Whenever the state changes from on hold the business rule should trigger and the duration of start to end should be added to the new field "On hold duration"n which is on sla table and it should plus the duration to the "On hold duration" field every time the state changes to on hold until it gets resolved.
