[http] Avoid boxing of status code tags

[CodeBlanch]

Changes

• Use a dictionary an array of pre-boxed status code values to avoid boxing perf hit on every request. Borrowed from AspNetCore's upcoming metric support.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Appropriate CHANGELOG.md files updated for non-trivial changes

[codecov]

Codecov Report

Merging #4361 (b4ba9dc) into main (b2fb873) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4361      +/-   ##
==========================================
+ Coverage   84.79%   84.82%   +0.02%     
==========================================
  Files         299      300       +1     
  Lines       12057    12065       +8     
==========================================
+ Hits        10224    10234      +10     
+ Misses       1833     1831       -2     

Impacted Files	Coverage Δ
...tp/Implementation/HttpHandlerDiagnosticListener.cs	74.22% <100.00%> (ø)
...ementation/HttpHandlerMetricsDiagnosticListener.cs	95.23% <100.00%> (ø)
...plementation/HttpWebRequestActivitySource.netfx.cs	80.23% <100.00%> (ø)
...rumentation.Http/Implementation/TelemetryHelper.cs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[reyang]

@JamesNK FYI.