Currently Manager.Start immediately returns when stop channel is closed without waiting for Runnables to stop:

This makes writing a Runnable with blocking stop logic difficult or impossible. For example, the Manager's internal serveMetrics is basically a Runnable (a blocking method that takes a stop channel). When the stop channel is closed, serveMetrics tries to shut down its http server:

But in normal usage, the process immediately exits when Manager.Start returns so the Shutdown call is unlikely to complete.

Adding sync.WaitGroup accounting inside the Runnable wrapper goroutines would allow the addition of a Wait() method to the Manager interface that blocks until all Runnables have returned (or WaitWithTimeout(time.Duration), Shutdown(context.Context) for a termination timeout), without changing either the Runnable contract or the Manager contract:

if err := mgr.Start(signals.SetupSignalHandler()); err != nil {
    log.Error(err, "unable to run the manager")
    os.Exit(1)
}
mgr.Wait()