1. A system comprising: a resource manager computer system to allocate computer resources from a pool of computer resources for executing an application; a data monitoring system to monitor metrics for the allocated computer resources; and a resource assessment computer system to receive and execute a control plug-in specifying a policy for determining whether to adjust the computer resources allocated to execute the application based on the metrics, wherein the control plug-in includes a control function specifying a calculation for calculating a new target amount of the computer resources that satisfies a desired performance of the computer resources under current system conditions based on the parameters, and the parameters include an operational condition and a desired condition of the computer resources, and the resource assessment computer system is to execute the control plug-in, including the control function, to calculate the new target amount of the computer resources based on the parameters, and the resource assessment computer system is to determine whether to adjust the computer resources by comparing the new target amount of the computer resources with the metrics, wherein the resource assessment computer system is to receive a new desired condition and include the new desired condition in the control plug-in to customize the control plug-in, and execute the customized control plug-in to calculate a revised target amount of the computer resources based on the parameters and the new desired condition.