Instrumenting Global module

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

I am unable to add instrumentation for Global Objects in the agent in a way that is described [here](https://github.com/elastic/apm-agent-nodejs/issues/312). There are few methods that I needed to instrument like `eval()`, `setTimeout()` etc.
(PS: I don't know if I am approaching this right)

**Describe the solution you'd like**
Instrumentation of Global objects should be either configurable or supported.

**Describe alternatives you've considered**
I have tried adding custom instrumentation by creating a `global.js` file in `lib/instrumentation/modules` and added it to [MODULES](https://github.com/elastic/apm-agent-nodejs/blob/dd20f63cfc56f3078c5e24e56a0f58a2d859a999/lib/instrumentation/index.js#L16) but the agent does not auto instrument this.

**Additional context**
For monkey-patching say eval, I've used shimmer to wrap `global` for eval function.
`shimmer.wrap(global, "eval", wrapEval)`

For reference - https://github.com/elastic/apm-agent-nodejs/issues/312