could we log.warn if `require.cache` already has modules we typically instrument at `agent.start()` time?

A reasonably common mistake when using the APM agent is to `.start()` it *after* having `require()`d some modules that the agent *would* have instrumented. We could probably look through `require.cache` at `Agent#start()`-time to see if it already had imported modules that the agent *would* have instrumented (modulo `active` and `disableInstrumentations`), and if so, then emit a `log.warn('yo, you need to apm.start() earlier')`. Then that log message could be in the troubleshooting docs to explain the issue.