Fix executor instrumentation by unordered config merging

[Forbiddensequence]

Fixes #1294

[mkurz]

@ivantopo can you please merge this? This is such a low hanging fruit...

[ivantopo]

Sorry guys, I know this looks like extremely low hanging fruit that should be handled faster. I didn't merge it yet because I remember having a similar issue many year ago and trying to use the += operator but that didn't work out. I vaguely remember having situations where the config wouldn't resolve because of adding to an array that doesn't exist, but that was so long ago and I can't find any traces of it.

So I thought I'll first try a few things in different types of applications and then merge it if I can't reproduce whatever issue I had in the past. I'll try to do that now while the tests run :D

[ivantopo]

I couldn't find any obvious way to break this so we are merging it and hoping it will all be fine 😄

[ivantopo]

Thank you @Forbiddensequence for the fix 👋