fix toString for duartion

[Nicole00]

No description provided.

[codecov-commenter]

Codecov Report

Merging #422 (ab9e41f) into master (eb5ae6b) will decrease coverage by 0.35%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #422      +/-   ##
============================================
- Coverage     63.65%   63.29%   -0.36%     
+ Complexity      730      725       -5     
============================================
  Files            66       66              
  Lines          3442     3444       +2     
  Branches        500      501       +1     
============================================
- Hits           2191     2180      -11     
- Misses          912      922      +10     
- Partials        339      342       +3     

Impacted Files	Coverage Δ
.../vesoft/nebula/client/graph/data/ValueWrapper.java	56.00% <50.00%> (-1.08%)	⬇️
...java/com/vesoft/nebula/client/meta/MetaClient.java	54.18% <0.00%> (-4.93%)	⬇️

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