Wakapi
Wakapi is an open-source server-side alternative to WakaTime. The clients are already mostly open-source.
The server is configured to forward all heartbeats to my upstream wakatime account.
This service is public. It is served by veil at https://wakapi.mzhang.io.
Registration is NOT open.
Client configuration
Make sure the Wakatime configuration file ($HOME/.wakatime.cfg) contains this:
[settings]api_url = https://wakapi.mzhang.io/apiapi_key = <api_key>