It has become a common requirement for testbeds to provide a service in charge of collecting and exposing metrics, thus assisting experimenters with the central task of data collection. This paper describes Kwollect, a service designed and developed in the context of the Grid'5000, to collect infrastructure metrics (including high-frequency wattmeters) and expose them to experimenters. Kwollect scales to high frequencies of metrics collection for hundreds of nodes. It can also be leveraged by the experimenter to collect custom metrics.