In one embodiment, webpage data for a webpage is downloaded by a web browser executed by a client device. The client device inserts instrumentation into the webpage data to collect event metrics for events associated with the webpage. The client device selects, based on a user-defined policy, a set of event metrics from among the collected event metrics to be shared with a proxy service. The client device sends the selected set of event metrics to the proxy service. The proxy service provides access to the set of event metrics to one or more collectors registered with the proxy service.