update according to review comments

[localvar]

• add version to spec
• change status code to 503 when no response
• add more debug log
• fix typo

[codecov-commenter]

Codecov Report

Merging #667 (680f323) into pipeline (d3d3783) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##           pipeline     #667      +/-   ##
============================================
+ Coverage     78.69%   78.71%   +0.01%     
============================================
  Files            93       93              
  Lines         10098    10100       +2     
============================================
+ Hits           7947     7950       +3     
+ Misses         1681     1679       -2     
- Partials        470      471       +1     

Impacted Files	Coverage Δ
pkg/filters/proxy/pool.go	61.98% <15.38%> (+1.16%)	⬆️
pkg/object/httpserver/mux.go	85.48% <72.22%> (-0.81%)	⬇️
pkg/filters/filters.go	69.38% <100.00%> (ø)

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