Use Cow<'static, str> for api::Value

[djc]

No description provided.

[codecov]

Codecov Report

Merging #332 into master will decrease coverage by 0.03%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   53.42%   53.38%   -0.04%     
==========================================
  Files          71       71              
  Lines        5818     5816       -2     
==========================================
- Hits         3108     3105       -3     
- Misses       2710     2711       +1     

Impacted Files	Coverage Δ
opentelemetry/src/api/baggage.rs	80.00% <ø> (ø)
opentelemetry/src/exporter/metrics/stdout.rs	0.00% <ø> (ø)
opentelemetry/src/api/core.rs	59.57% <87.50%> (+1.33%)	⬆️
opentelemetry/src/sdk/env.rs	75.00% <91.66%> (-4.07%)	⬇️
opentelemetry/src/sdk/propagation/baggage.rs	96.42% <100.00%> (ø)
opentelemetry/src/sdk/resource.rs	77.57% <100.00%> (-0.42%)	⬇️

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