Per discussion in T219435: Homepage: HomepageModule schema, implement a sampling rate for hover-in/hover-out events on Special:Homepage.

The sampling rate should be defined as a configuration variable, and should be used to sample X percent of visits to Special:Homepage. If in the sample, then all hover-in/hover-out events are recorded in HomepageModule schema. If not in the sample, no hover-in/hover-out events are recorded in HomepageModule schema. Whether the page request is in sample is determined on the server-side, and is logged in the "impression" event for each module on the client-side.