docs: Modify examples to show best practices - reuse tracer

[cijothomas]

One of many with the goal of showcasing best practices in all examples maintained in this repo.
This PR shows how to re-use Tracer instead of repeatedly obtaining it from provider/global which involve at-least one atomic check and an Arc clone internally.
A simple benchmark was added to show the impact of Tracer creation.

If the direction is agreed, we need to review every examples and make sure they show best practices for all signals.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.3%. Comparing base (11ed8e0) to head (dd61331).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2709   +/-   ##
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  Coverage   79.3%   79.3%           
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  Files        123     123           
  Lines      22670   22670           
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  Hits       17986   17986           
  Misses      4684    4684           

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[lalitb]

Nice.