Also run Go garbage collection as part of Usage.Reduce.

The `Usage.Reduce` RPC's promise is to reclaim as much Sentry memory as
possible and to give it back to the host.
Seems like running Go GC should be part of that.

PiperOrigin-RevId: 547312564

pkg/sentry/control/usage.go

@@ -142,6 +142,8 @@ func (u *Usage) Reduce(opts *UsageReduceOpts, out *UsageReduceOutput) error {
 	if opts.Wait {
 		mf.WaitForEvictions()
 	}
+	// Also reduce Sentry memory usage by garbage-collecting now.
+	runtime.GC()
 	return nil
 }