feat(starfish): add http.request.method attribute to span data

[aldy505]

Closes #713

Since existing performance tracing only available on net/http and gin, I only implement http.request.method attribute on those.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.97%. Comparing base (a084ac9) to head (e1010d7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #786      +/-   ##
==========================================
- Coverage   82.04%   81.97%   -0.08%     
==========================================
  Files          49       49              
  Lines        4846     4848       +2     
==========================================
- Hits         3976     3974       -2     
- Misses        729      732       +3     
- Partials      141      142       +1     

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[aldy505]

I suppose there'll be a merge conflict, but let's see