Bug fix: metrics panic when request is invalid.

[little-cui]

No description provided.

[coveralls]

Coverage increased (+0.05%) to 71.721% when pulling 087263c on little-cui:master into 2486c78 on ServiceComb:master.

[codecov-io]

Codecov Report

Merging #211 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   67.99%   67.99%           
=======================================
  Files          17       17           
  Lines        3584     3584           
=======================================
  Hits         2437     2437           
  Misses        974      974           
  Partials      173      173

Impacted Files	Coverage Δ
server/service/service_dependency.go	76.52% <0%> (-0.87%)	⬇️
server/service/tag.go	74.41% <0%> (+0.58%)	⬆️

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