Mobile Client SDKs. Free and open source. Log events like clicks, IAB impressions, dwell-time, and attributed conversions. Events are standardized and cross-platform. Some events like "impression" are actually a stream of visibility events that depend on the device and app. While the Client Metrics SDK may do some event processing, the Client Metrics SDKs assume that complex processing happens on the server.
Client Metrics SDKs are a part of the overall Metrics SDK. See Server SDK for server-side Metrics SDK components.