test: add unit-tests for NilMetrics

[jizhuozhi]

Issues associated with this PR

Add test cases for #2310

Solutions

You should show your solutions about the issues in your PR, including the overall solutions, details and the changes. At this time, Chinese is allowed to describe these.

UT result

Unit Test is needed if the code is changed, your unit test should cover boundary cases, corner cases, and some exceptional cases. And you need to show the UT result.

Benchmark

If your code involves the processing of every request, you should give the Benchmark Result.

Code Style

• Make sure Goimports has run
• Show Golint result

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.06 ⚠️

Comparison is base (dcb0ff1) 60.54% compared to head (d016e04) 60.49%.

❗ Current head d016e04 differs from pull request most recent head 80eeffe. Consider uploading reports for the commit 80eeffe to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2312      +/-   ##
==========================================
- Coverage   60.54%   60.49%   -0.06%     
==========================================
  Files         422      422              
  Lines       37477    37477              
==========================================
- Hits        22691    22670      -21     
- Misses      12549    12570      +21     
  Partials     2237     2237              

see 6 files with indirect coverage changes

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