feat: allow manual instrumentation when instrument is set to false

[Qard]

Fixes #858

Checklist

• Implement code
• Add tests
• Update documentation

[watson]

We should probably put this in v3.0.0 as it could be considered a breaking change 🤔

[Qard]

True. Marking as breaking change.

[watson]

Jenkins run the tests please

[watson]

I'm able to reproduce the Travis failure locally. The issue is that the CPU just starts spinning forever during the test/config.js test. It's happening on the latest Node.js v10, but not v12 when running nyc node test/config.js, but doesn't happen when running node test/config.js. So something that nyc is doing is causing something.

When it occurs, the process hangs so badly that you need to run a kill -9 to exit it.

[watson]

It's now Monday, and I upgraded to Safari 13 over the weekend, which required me to agree on a new XCode License Agreement before git would work in the terminal again. And I'm no longer able to reproduce the error locally

[watson]

Ok, I can reproduce it again - but not every time. Turns out the error doesn't always happen, so sometimes you need to run the test command a few times before it occurs. So I usually run it like this:

while true; do ./node_modules/.bin/nyc node test/config.js || break; done

I made a core dump while the script was stuck in the infinite loop and discovered that it was related to async hooks. When disabling async hooks in the instrument: false allows manual instrumentation test, the error disappeared. This obviously isn't a solution but might help in determining the cause.

[watson]

I think I narrowed it down to multiple async hooks being enabled during the test/config.js tests. If I put the return value of asyncHooks.createHook() into a global variable on which I call .disable() if it's already present when trying to create a new hook, then the problem goes away.

Working on a fix...

[watson]

I've fixed the CI issue in #1373. Once that's merged, master needs to be merged into this PR and it should hopefully be ready to merge

[watson]

I think you accidentally force pushed a version that removed my last commit. I assume that was just because you rebased with master without pulling first?

I'll force push a new version with the extra commits. Hope you don't mind 🙂

[watson]

jenkins run the tests please

[Qard]

Ah, sorry about that. I missed your commits. 😅

[Qard]

jenkins run the tests please