Use correct semantic conventions for trace labels

[sfleen]

The OpenTelemetry spec defines the semantic conventions that HTTP services should use for the labels included in traces: https://opentelemetry.io/docs/specs/semconv/http/http-spans/

Previously, we were using an outdated version of this spec for the OpenCensus traces. This updates the labels to match the current spec.

The notable changes updates to the path for HTTP method and status code, the fields that include the URL parts, and more rigorously following the standard for propagating the Host header.

[cratelyn]

this looks good! i have two really minor nits to offer, but the change itself seems great 🙂

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.

Project coverage is 66.78%. Comparing base (96124bc) to head (470a701).
Report is 688 commits behind head on main.

Files with missing lines	Patch %	Lines
linkerd/trace-context/src/service.rs	0.00%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3484      +/-   ##
==========================================
- Coverage   67.68%   66.78%   -0.90%     
==========================================
  Files         332      388      +56     
  Lines       15158    18155    +2997     
==========================================
+ Hits        10259    12125    +1866     
- Misses       4899     6030    +1131     

Files with missing lines	Coverage Δ
linkerd/trace-context/src/service.rs	16.66% <0.00%> (-3.34%)	⬇️

... and 173 files with indirect coverage changes

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