config: add support for configuring propagators

[jpkrohling]

Fixes #6712

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.

Project coverage is 74.5%. Comparing base (d21dd87) to head (7d7a2fc).

Files with missing lines	Patch %	Lines
config/v0.3.0/config.go	57.1%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6727     +/-   ##
=======================================
- Coverage   74.5%   74.5%   -0.1%     
=======================================
  Files        199     200      +1     
  Lines      17529   17551     +22     
=======================================
+ Hits       13075   13091     +16     
- Misses      4070    4074      +4     
- Partials     384     386      +2     

Files with missing lines	Coverage Δ
config/v0.3.0/propagation.go	100.0% <100.0%> (ø)
propagators/autoprop/registry.go	100.0% <100.0%> (ø)
config/v0.3.0/config.go	86.1% <57.1%> (-2.4%)	⬇️

... and 1 file with indirect coverage changes

[pellared]

Changelog entry is missing.

[codeboten]

Thanks for taking on this work @jpkrohling! Added a couple of comments

[codeboten]

It might be possible to simplify the code here by using the autoprop package

[pellared]

Given the second reviewer says the same I think this could be addressed

Reference: #6727 (comment)

[jpkrohling]

Done! I'm not sure the current code justifies a new function and new source file. On the other hand, it's aligned with how things are done for similar constructs. I think I prefer this way, but I don't have strong opinions.

[jpkrohling]

Note: I changed a couple of things in autoprop, to account for two edge cases: an empty input, which IMO should have the same behavior as if the value wasn't specified), and an empty value, which IMO should be skipped.