Update KeyValue to avoid copying non-static string slices

[utpilla]

Related to #1642

Changes

• Update Value struct to avoid copying non-static string slices

Benchmarks

• There is around a 32% improvement in perf for the scenario where the attribute values are not static string slices
• No regression with other existing scenarios 

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[utpilla]

@cijothomas The changes in this PR are helpful for this scenario.

[cijothomas]

Can you add metric e2e benchmarks too? the counter.add and friends

[codecov]

Codecov Report

Attention: Patch coverage is 64.20455% with 63 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (66f29b8) to head (91f0a1d).

Files with missing lines	Patch %	Lines
opentelemetry/src/common.rs	68.5%	11 Missing ⚠️
opentelemetry-proto/src/transform/common.rs	11.1%	8 Missing ⚠️
opentelemetry/src/baggage.rs	63.6%	8 Missing ⚠️
opentelemetry-stdout/src/common.rs	0.0%	7 Missing ⚠️
opentelemetry/src/metrics/noop.rs	0.0%	6 Missing ⚠️
opentelemetry/src/trace/noop.rs	0.0%	4 Missing ⚠️
opentelemetry-proto/src/transform/metrics.rs	0.0%	2 Missing ⚠️
opentelemetry/src/global/trace.rs	66.6%	2 Missing ⚠️
opentelemetry/src/metrics/instruments/counter.rs	33.3%	2 Missing ⚠️
opentelemetry/src/metrics/instruments/gauge.rs	33.3%	2 Missing ⚠️
... and 9 more

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2089   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        121     121           
  Lines      20969   20960    -9     
=====================================
+ Hits       16335   16347   +12     
+ Misses      4634    4613   -21     

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[cijothomas]

1. Would love to add benchmarks for Metrics to see the overall gains in metric hot path.
2. Given this is doable without disrupting users, I'd prefer to park this and get back post RC stage, so we can focus on fixing critical bugs/feature gaps now.

As usual, great work! Thanks for constantly pursuing to get the highest performance. From my knowledge of other OTel clients, none of them allow alloc-free recordings when the key/values are a slice/part of an existing string, so this is a great improvement!