feat: improve writev encoding performance

[Qard]

Rather than using reduce and string concatentation, which creates many intermediary strings, use map and join instead which only creates an intermediary array and a single string allocation

This may fix issue #80.

[watson]

I updated the PR description so that it doesn't automatically close the referenced issue as you wrote may. Thought that was better and allows you to manually close it if it works

[watson]

FYI: I've released this in v9.0.0. Unless we make an 8.x branch and backport it to that, this will only make it into v3.x of the agent. But I don't deem this an important fix that requires us to backport it as it's a pretty crazy edge case that probably means you're using the agent wrong in the first place