Narrow ExtendedSpanBuilder return types for chaining

[trask]

Ran into this when updating open-telemetry/opentelemetry-java-examples#421

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.63%. Comparing base (7ee367a) to head (d3f9e16).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6514      +/-   ##
============================================
- Coverage     90.65%   90.63%   -0.03%     
- Complexity     6227     6228       +1     
============================================
  Files           678      679       +1     
  Lines         18659    18661       +2     
  Branches       1842     1842              
============================================
- Hits          16916    16914       -2     
- Misses         1187     1189       +2     
- Partials        556      558       +2     

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[jkwatson]

looks like you need to run the formatter on this to get rid of an unused import