impl(pubsub): start the publish span as a root span

[alevenberg]

Now that open-telemetry/opentelemetry-cpp#2427 is in we can start an async span as a root span

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This change is 

[alevenberg]

Since this was added recently, it won't be available in the oldest deps. https:/open-telemetry/opentelemetry-cpp/blame/main/api/include/opentelemetry/trace/span_metadata.h#L24. Gonna do something hacky and just use the string until we upgrade

Opened #13287

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f4b0970) 93.05% compared to head (f02ead8) 93.05%.
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13285      +/-   ##
==========================================
- Coverage   93.05%   93.05%   -0.01%     
==========================================
  Files        2137     2137              
  Lines      186491   186517      +26     
==========================================
+ Hits       173548   173564      +16     
- Misses      12943    12953      +10     

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