DeviceUsageDuration: How much the device resource is used to perform the test.
This is the device usage used for billing purpose, which is different from the run_duration, for example, infrastructure failure won't be charged for device usage.
PRECONDITION_FAILED will be returned if one attempts to set a device_usage on a step which already has this field set.
- In response: present if previously set. - In create request: optional - In update request: optional