1. A computer program product for measuring resource usage in a computing environment, said computer program product comprising: a computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: executing a Modify PCI Function Controls (MPFC) instruction comprising a function handle for identifying a PCI function of a PCI adapter of the computing environment on behalf of which the MPFC instruction is being performed, the MPFC instruction being a measurement registration instruction and specifying a location in memory for tracking information for the PCI function, and wherein the function handle comprises a function number identifying the PCI function, the function number configured to address the PCI function and to index into a function table that includes one or more characteristic of the PCI function; determining that the PCI function is being accessed via an instruction or requesting access of system memory coupled to the PCI function; tracking information specific to the PCI function, wherein the tracked information comprises a count of determined accessing instructions or a count of determined requested accesses to memory, and wherein the tracked information is on a per-function basis; and storing the tracked information in the location in memory.