feat: Add config metrics for Sentinel instances

[destinyoooo]

Add support for sentinel config metrics via sentinel config get *
Fix issue #841

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.86%. Comparing base (02ad204) to head (ef9d2f6).

Files with missing lines	Patch %	Lines
exporter/sentinels.go	47.82%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1058      +/-   ##
==========================================
- Coverage   82.17%   81.86%   -0.31%     
==========================================
  Files          20       20              
  Lines        3136     2636     -500     
==========================================
- Hits         2577     2158     -419     
+ Misses        443      358      -85     
- Partials      116      120       +4     

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[coveralls]

Pull Request Test Coverage Report for Build 18934570916

Details

• 22 of 32 (68.75%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 85.741%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
exporter/sentinels.go	18	28	64.29%

Totals
Change from base Build 18738909720:	-0.2%
Covered Lines:	2724
Relevant Lines:	3177

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💛 - Coveralls

[destinyoooo]

Furthermore, I found that the test can be controlled by configuring the InclConfigMetrics parameter. So I update test case
@oliver006