test: cover more cases

[yuanbohan]

What's changed and what's your intention?

increase code coverage

Checklist

• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

#33

[codecov]

Codecov Report

Merging #69 (c988bec) into main (85d0248) will increase coverage by 5.44%.
The diff coverage is 73.07%.

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   79.53%   84.97%   +5.44%     
==========================================
  Files          11       11              
  Lines         767      772       +5     
==========================================
+ Hits          610      656      +46     
+ Misses        119       85      -34     
+ Partials       38       31       -7     

Impacted Files	Coverage Δ
client.go	59.09% <ø> (+12.42%)	⬆️
series.go	86.89% <62.50%> (+15.17%)	⬆️
metric.go	80.70% <75.00%> (+2.80%)	⬆️
config.go	87.23% <100.00%> (-12.77%)	⬇️

... and 2 files with indirect coverage changes

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