Use Duration.Truncate for truncating precision

[y-yagi]

Duration.Truncate was added in Go 1.9 and Gin required Go version 1.13+ now.
So we can use Duration.Truncate.

• With pull requests:
  • Open your pull request against master
  • Your pull request should have no more than two commits, if not you should squash them.
  • It should pass all tests in the available continuous integration systems such as TravisCI.
  • You should add/modify tests to cover your proposed code changes.
  • If your pull request contains a new feature, please document it on the README.

[codecov]

Codecov Report

Merging #2711 (ec030a0) into master (5452a1d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2711   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          41       41           
  Lines        2036     2036           
=======================================
  Hits         2009     2009           
  Misses         15       15           
  Partials       12       12           

Impacted Files	Coverage Δ
logger.go	100.00% <100.00%> (ø)

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[thinkerou]

lgtm