Fix missing attributes in OpenTelemetry traces

[sfleen]

A previous change enabling OpenTelmetry forgot to include attributes from the span itself, in addition to the request level spans as we do in the old OpenCensus exporter. This adds those attributes back in.

[olix0r]

When merging this, please use a conventional commit summary. For example:

fix(opentelemetry): preserve span labels in traces

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.80%. Comparing base (96124bc) to head (65cdd9d).
Report is 794 commits behind head on main.

Files with missing lines	Patch %	Lines
linkerd/opentelemetry/src/lib.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3609      +/-   ##
==========================================
- Coverage   67.68%   66.80%   -0.88%     
==========================================
  Files         332      392      +60     
  Lines       15158    18414    +3256     
==========================================
+ Hits        10259    12301    +2042     
- Misses       4899     6113    +1214     

Files with missing lines	Coverage Δ
linkerd/opentelemetry/src/lib.rs	0.00% <0.00%> (ø)

... and 171 files with indirect coverage changes

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