Change default trace protocol to OpenTelemetry

[sfleen]

Now that OpenTelemetry has been available for a few releases, we should enable it as the default to begin to phase out OpenCensus.

This sets the default protocol for the proxy, we'll need to make the corresponding changes to the control plane as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.78%. Comparing base (96124bc) to head (a472c26).
Report is 664 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3469      +/-   ##
==========================================
- Coverage   67.68%   66.78%   -0.90%     
==========================================
  Files         332      388      +56     
  Lines       15158    18139    +2981     
==========================================
+ Hits        10259    12114    +1855     
- Misses       4899     6025    +1126     

Files with missing lines	Coverage Δ
linkerd/app/core/src/http_tracing.rs	17.39% <ø> (+6.07%)	⬆️

... and 168 files with indirect coverage changes

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[olix0r]

Doesn't the control plane always have to tell the proxy to use the opentelemetry protocol?

This change, aiui, would cause proxies to default to opentelemetry when running with an older control plane that does not know to set this default.

I think we do not want to change the proxy's defaulting behavior with this in mind: the decision should be driven/controlled by the injector.