func (h *StandardHealthcheck) Unhealthy(err error) {
	h.err = err
}
Unhealthy marks the healthcheck as unhealthy.  The error is stored and
may be retrieved by the Error method.
package metrics
type Healthcheck interface {
	Check()
	Error() error
	Healthy()
	Unhealthy(error)
}
func NewHealthcheck(f func(Healthcheck)) Healthcheck {
	if UseNilMetrics {
		return NilHealthcheck{}
	}
	return &StandardHealthcheck{nil, f}
}
type NilHealthcheck struct{}
func (NilHealthcheck) Check() {}
func (NilHealthcheck) Error() error { return nil }
func (NilHealthcheck) Healthy() {}
func (NilHealthcheck) Unhealthy(error) {}
type StandardHealthcheck struct {
	err error
	f   func(Healthcheck)
}
func (h *StandardHealthcheck) Check() {
	h.f(h)
}
func (h *StandardHealthcheck) Error() error {
	return h.err
}
func (h *StandardHealthcheck) Healthy() {
	h.err = nil
}