code: func (r *RegisterMonitor) heartbeat() {
  	// Trigger the health check passing. We don't need to retry this
  	// since we do a couple tries within the TTL period.
  	if err := r.Client.Agent().PassTTL(r.checkID(), ""); err != nil {
  		if !strings.Contains(err.Error(), "does not have associated") {
  			r.Logger.Printf("[WARN] proxy: heartbeat failed: %s", err)
  		}
  	}
  }
docs: // heartbeat just pings the TTL check for our service.