Remove Option from Array values (fixes #57)

[djc]

As mandated by the spec change in open-telemetry/opentelemetry-specification#1214.

[codecov]

Codecov Report

Merging #359 (bd57056) into master (769592b) will decrease coverage by 0.01%.
The diff coverage is 30.00%.

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
- Coverage   54.21%   54.19%   -0.02%     
==========================================
  Files          70       70              
  Lines        5849     5836      -13     
==========================================
- Hits         3171     3163       -8     
+ Misses       2678     2673       -5     

Impacted Files	Coverage Δ
opentelemetry/src/api/labels/mod.rs	0.00% <0.00%> (ø)
opentelemetry/src/api/core.rs	54.62% <33.33%> (-1.11%)	⬇️

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