ensure zipkin tags and attributes are cast to strings

[brettmc]

Closes: #1404

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.77%. Comparing base (2c1da9a) to head (f6030dd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1406      +/-   ##
============================================
+ Coverage     73.68%   73.77%   +0.09%     
  Complexity     2682     2682              
============================================
  Files           387      387              
  Lines          7672     7672              
============================================
+ Hits           5653     5660       +7     
+ Misses         2019     2012       -7     

Flag	Coverage Δ
8.1	73.41% <100.00%> (+0.01%)	⬆️
8.2	73.73% <100.00%> (+0.11%)	⬆️
8.3	73.63% <100.00%> (ø)
8.4	73.65% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
src/Contrib/Zipkin/SpanConverter.php	97.16% <100.00%> (+0.70%)	⬆️

... and 2 files with indirect coverage changes

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