Is there an established metric to describe optimal/needed ressources for a specific computational task?
As far as I'm aware, there is no universal standard amongst cloud providers. One reason for this is because most cloud providers like to abstract how their backend infrastructure, that their instances run on, is managed. How your instance is ran could change minute by minute depending on the cloud provider. The user doesn't care as long as they are getting the desired computing power that they want. Cloud providers also may not want to divulge their exact infrastructure metrics for proprietary reasons as well. 

Despite these reasons, some cloud providers offer a standard amongst their own virtual instances. These standards are relative to the computing power of each instance type that the provider offers.

For example, Amazon Web Services uses EC2 Computing Units (ECU) as a metric to compare the approximate computing power in relation to each instance. Comparing two instances types such as an m5.large and an m5.xlarge for example, the m5.large has an ECU of 8 vs the m5.xlarge which has a ECU of 16. The m5.xlarge has roughly double the compute power and therefore double the cost.