test: add UT for gRPC Encoder/Decoder

[xingfudeshi]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

add UT for gRPC Encoder/Decoder

Ⅱ. Does this pull request fix one issue?

fixes #7206

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.61%. Comparing base (44dd9d6) to head (ccdc0aa).
Report is 2 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #7221      +/-   ##
============================================
+ Coverage     53.46%   53.61%   +0.15%     
- Complexity     7109     7115       +6     
============================================
  Files          1169     1169              
  Lines         41578    41578              
  Branches       4873     4873              
============================================
+ Hits          22229    22292      +63     
+ Misses        17250    17177      -73     
- Partials       2099     2109      +10     

see 10 files with indirect coverage changes

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[slievrly]

LGTM