Consider adding apm.isActive() method

**Is your feature request related to a problem? Please describe.**

Trying to work around #1799, I thought I would first check whether the agent is active before calling `registerMetric()` but it seems there's no API to detect that. The `isStarted()` method returns `true` if `start()` had been called, regardless of the active status. Even when elastic/apm-agent-nodejs#1799 gets fixes, it may be useful to have an `isActive()` method.