fix(propagator-b3): update extract to check for array

[jordanworner]

Which problem is this PR solving?

• Fixes GRPC instrumentation doesn't extract from B3 multi propagator #2147, the TextMapGetter in opentelemetry-instrumentation-grpc returns an empty array if the header value cannot be found. The extract method in B3Propagator only checks for a truthy value and will fail to extract b3 multi headers.

Short description of the changes

• Updated the check for the b3 single header to ensure the value is not an empty array.
• Added a test to check a b3 multi header is extracted if the TextMapGetter returns an array.

[dyladan]

What truthy value causes this check to pass which is avoided by checking for .length? To me this is quite unintuitive. I would rather see Array.isArray() or typeof === 'string' or something which makes intent more clear.

[codecov]

Codecov Report

Merging #2285 (81de1e2) into main (b442048) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head 81de1e2 differs from pull request most recent head 7511b48. Consider uploading reports for the commit 7511b48 to get more accurate results

@@            Coverage Diff             @@
##             main    #2285      +/-   ##
==========================================
+ Coverage   92.74%   92.76%   +0.02%     
==========================================
  Files         145      145              
  Lines        5182     5184       +2     
  Branches     1059     1060       +1     
==========================================
+ Hits         4806     4809       +3     
+ Misses        376      375       -1     

Impacted Files	Coverage Δ
...es/opentelemetry-propagator-b3/src/B3Propagator.ts	100.00% <100.00%> (ø)
...emetry-core/src/platform/node/RandomIdGenerator.ts	93.75% <0.00%> (+6.25%)	⬆️

[jordanworner]

@dyladan I have updated it and normalized the header value so the intention is clear. An empty array evaluates to true so it was passing the check.

[obecny]

lgtm