config: add support for configuring propagators

[jpkrohling]

Fixes #6712

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.6%. Comparing base (8b5220b) to head (b61af11).
Report is 12 commits behind head on main.

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   75.5%   75.6%           
=====================================
  Files        207     208    +1     
  Lines      19152   19177   +25     
=====================================
+ Hits       14478   14500   +22     
- Misses      4239    4241    +2     
- Partials     435     436    +1     

Files with missing lines	Coverage Δ
config/v0.3.0/propagation.go	100.0% <100.0%> (ø)
config/v0.3.0/config.go	86.3% <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

[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.

[jpkrohling]

If this looks good, I'll fix the merge conflicts.

[MrAlias]

Looks good overall. Thanks for working on this.

[pellared]

There is a difference between

 "propagator": { 
 }, 

and

 "propagator": { 
     "composite": [] 
 }, 

From the specification:

If a property has a default value defined (i.e. is not required) and is missing or present but null, Create MUST ensure the SDK component is configured with the default value.

The first one should return the default value. The second an empty composite propagator.
The code should be probably like this:

	if cfg.opentelemetryConfig.Propagator.Composite == nil {
		return propagation.NewCompositeTextMapPropagator()
	}
	n := len(cfg.opentelemetryConfig.Propagator.Composite)
	if n == 0 {
		return autoprop.NewTextMapPropagator(), nil
	}

[pellared]

I do not see anything in the schema that disallows using empty name. See: https://github.com/open-telemetry/opentelemetry-configuration/blob/main/schema/propagator.json

autoprop supports empty names (similarly to database/sql).

Sorry that I was not precise in #6727 (comment) 🙇

[codeboten]

i opened an issue in the configuration repo to discuss this, i submitted a PR to propose enforcing a minimum length on propagator configuration