func (m *Metrics) Start(name string) {
	if m == nil {
		return
	}
	m.closeMark()
	m.curMark = &mark{name: name}
	// Unlikely we need to a GC here, as one was likely just done in closeMark.
	if m.shouldPProf() {
		f, err := os.Create(makePProfFilename(m.filebase, name, "cpuprof"))
		if err != nil {
			panic(err)
		}
		m.pprofFile = f