1. A system, comprising: a plurality of compute nodes that together implement a provider network, wherein each of the plurality of compute nodes comprise at least one processor and a memory; at least some of the plurality of compute nodes implement respective virtualization hosts for a plurality of virtual compute instances implemented as part of the provider network; a control plane for the provider network, the control plane configured to: maintain a resource credit pool comprising a plurality of resource credits available to replenish an individual resource credit balance for one or more virtual compute instances that are authorized to obtain resource credits from the resource credit pool of the plurality of virtual compute instances, wherein the plurality of resource credits are individually expendable to increase utilization of a physical computer resource for an individual one of the one or more authorized virtual compute instances in excess of a utilization guarantee for the physical computer resource at one of the respective virtualization hosts until the resource credit is expended; receive a resource credit request for a virtual compute instance of the one or more authorized virtual compute instances to replenish the individual resource credit balance for the virtual compute instance; in response to the receipt of the resource credit request: determine a number of resource credits to add to the individual resource credit balance for the virtual compute instance; send a response indicating the number of resource credits to be added to the individual resource credit balance for the virtual compute instance; and remove the calculated number of resource credits from the resource credit pool.