make Tracer::Clone cheaper (another 10% improvement to start-end-span/never-sample)

[shaun-cox]

Fixes #1092

This is more of a workaround in that I think there is still more work to be done inside InstrumentationLibrary type itself, but waiting for more discussion.

Changes

• change Tracer to hold an Arc rather than the whole 120 byte instance, so that Cloning is way cheaper
• IMO, the Arc should move internal to InstrumentationLibrary itself, but that is a bigger API-breaking change.

start-end-span/always-sample
                        time:   [508.72 ns 523.52 ns 536.82 ns]
                        change: [-6.7810% -1.9044% +3.2262%] (p = 0.46 > 0.05)
                        No change in performance detected.
start-end-span/never-sample
                        time:   [120.16 ns 120.48 ns 120.81 ns]
                        change: [-10.510% -10.308% -10.116%] (p = 0.00 < 0.05)
                        Performance has improved.

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)

[codecov]

Codecov Report

Patch coverage: 100.0% and no project coverage change.

Comparison is base (800b9e3) 50.7% compared to head (8af3715) 50.7%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1093   +/-   ##
=====================================
  Coverage   50.7%   50.7%           
=====================================
  Files        165     165           
  Lines      19801   19808    +7     
=====================================
+ Hits       10051   10059    +8     
+ Misses      9750    9749    -1     

Impacted Files	Coverage Δ
opentelemetry-sdk/src/trace/provider.rs	90.0% <100.0%> (+0.7%)	⬆️
opentelemetry-sdk/src/trace/tracer.rs	93.5% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

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