autoprop: Handle empty env var as unset

[pellared]

Why

https://github.com/open-telemetry/opentelemetry-specification/blob/v1.23.0/specification/configuration/sdk-environment-variables.md?plain=1#L19

The SDK MUST interpret an empty value of an environment variable the same way as when the variable is unset.

What

Handle empty value of OTEL_PROPAGATORS environment variable the same way as when the variable is unset

Testing

The returned result of autoprop.NewTextMapPropagator has not changed. The only difference is that we do not log an unnecessary error via

opentelemetry-go-contrib/propagators/autoprop/propagator.go

Lines 53 to 54 in 3061fc5

	// Communicate to the user their supplied value will not be used.
	otel.Handle(err)

I am not sure it it is worth to add a unit test for it.

[codecov]

Codecov Report

Merging #4101 (f113a83) into main (3061fc5) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4101     +/-   ##
=======================================
- Coverage   79.2%   79.1%   -0.1%     
=======================================
  Files        165     165             
  Lines      10318   10318             
=======================================
- Hits        8173    8170      -3     
- Misses      2008    2010      +2     
- Partials     137     138      +1     

Files Changed	Coverage Δ
propagators/autoprop/propagator.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[MrAlias]

Doesn't this mean that if a user does something like OTEL_PROPAGATORS="$MY_RESOLVED_PROPS", but $MY_RESOLVED_PROPS is undefined the code will silently drop the error and just choose something for the user?

[pellared]

Doesn't this mean that if a user does something like OTEL_PROPAGATORS="$MY_RESOLVED_PROPS", but $MY_RESOLVED_PROPS is undefined the code will silently drop the error and just choose something for the user?

Correct. This is the expected behavior. More context: open-telemetry/opentelemetry-specification#2045