fix(srvhttp): RequestDurationSeconds shouldn't panic when missing labels

[Reasno]

No description provided.

[codecov]

Codecov Report

Merging #207 (c61c7ab) into master (ab0fffc) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   82.14%   82.17%   +0.03%     
==========================================
  Files          90       90              
  Lines        3445     3451       +6     
==========================================
+ Hits         2830     2836       +6     
  Misses        460      460              
  Partials      155      155              

Impacted Files	Coverage Δ
srvhttp/metrics.go	83.78% <100.00%> (+3.13%)	⬆️

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