Don't consider unset env var to be an error during resource detection

[matej-g]

In line with the suggestion in the issue #1114 and considering the text of the specification with regards to resource detection

Note the failure to detect any resource information MUST NOT be considered an error, whereas an error that occurs during an attempt to detect resource information SHOULD be considered an error.

The failure to set environment variable for OTEL_RESOURCE_ATTRIBUTES should not result in an error.

Resolves #1170

[codecov]

Codecov Report

Merging #1170 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1170     +/-   ##
========================================
- Coverage    78.2%   77.9%   -0.4%     
========================================
  Files         135     135             
  Lines        7163    7192     +29     
========================================
  Hits         5603    5603             
- Misses       1317    1346     +29     
  Partials      243     243             

Impacted Files	Coverage Δ
sdk/resource/auto.go	0.0% <ø> (ø)
sdk/resource/env.go	100.0% <ø> (ø)
label/type_string.go	12.5% <0.0%> (-37.5%)	⬇️
sdk/export/metric/exportkind_string.go	64.2% <0.0%> (-35.8%)	⬇️
api/metric/kind_string.go	16.6% <0.0%> (-33.4%)	⬇️
api/metric/numberkind_string.go	25.0% <0.0%> (-25.0%)	⬇️