feat: using Cow<'static, str> for instrumentation library.

[TommyCpp]

It allows users to pass in a String if needed, although it will have a performance overhead because we need to clone the string every time a span gets exported. I did a quick test and it seems the overhead is manageable. Less than 10%

Users that pass a 'static str will not be impacted as Cow::Borrow just copies the pointer.

Fix #666

[codecov]

Codecov Report

Merging #670 (1f632f7) into main (f6f69c4) will decrease coverage by 0.02%.
The diff coverage is 70.21%.

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
- Coverage   63.43%   63.41%   -0.03%     
==========================================
  Files          95       95              
  Lines        7784     7796      +12     
==========================================
+ Hits         4938     4944       +6     
- Misses       2846     2852       +6     

Impacted Files	Coverage Δ
opentelemetry/src/global/trace.rs	17.72% <0.00%> (-2.45%)	⬇️
opentelemetry-datadog/src/exporter/model/v05.rs	90.56% <66.66%> (-1.60%)	⬇️
opentelemetry/src/metrics/config.rs	50.00% <66.66%> (+5.17%)	⬆️
opentelemetry/src/sdk/trace/provider.rs	83.22% <75.00%> (+0.21%)	⬆️
opentelemetry/src/metrics/descriptor.rs	60.37% <80.00%> (+1.55%)	⬆️
opentelemetry-datadog/src/exporter/model/v03.rs	91.66% <100.00%> (ø)
opentelemetry-zipkin/src/exporter/model/mod.rs	82.53% <100.00%> (+0.87%)	⬆️
opentelemetry/src/metrics/meter.rs	100.00% <100.00%> (ø)
opentelemetry/src/sdk/instrumentation.rs	100.00% <100.00%> (ø)
opentelemetry/src/sdk/trace/span.rs	95.57% <100.00%> (ø)
... and 4 more

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[jtescher]

Looks good!