isStarted() returns true even if agent is not active

Describe the bug isStarted() returns true even if agent is not active

To Reproduce Steps to reproduce the behavior:

	const apm = require('elastic-apm-node').start({ active: false, ...});
	if (apm.isStarted()) { 
		apm.captureError(new Error(), err => console.error(err));
	}

Error logged: Error: cannot capture error before agent is started

Expected behavior Expect to use isStarted() as a guard to run captureError method

Environment (please complete the following information)

• OS: mac-os
• Node.js version: v8.11.2
• Agent version: lastic-apm-node@2.11.5

How are you starting the agent? (please tick one of the boxes)

• Calling agent.start() directly (e.g. require('elastic-apm-node').start(...))
• Requiring elastic-apm-node/start from within the source code
• Starting node with -r elastic-apm-node/start

[isitinme]

Could you please expose something like

isActive() {
    return this._conf.active;
}

to legitimize the usage to that config flag

[watson]

@sometimesalready Ah yes good point - I didn’t think of that. We need to ensure it doesn’t return an error in this case. Maybe we should also consider only having isStarted() return true if the agent is actually active - but that’s another issue. If not we should at least update the docs to better specify what to expect