feat: Make opentelemetry optional

[popzxc]

Fixes #564

Not all projects require opentelemetry, and besides being extra code to compile, it also makes supply chain security harder (see example).

I've kept opentelemetry enabled by default so that the change is backward compatible, hope that works.

[popzxc]

@tikue any chance you can look into it?

[tikue]

Hi, thanks for the contribution! I will try to look at this in the next week or so.

[popzxc]

@tikue sorry for pinging, would really appreciate if you'd find a minute to review the PR :)