fix CountDownLatch wait timeout in MetricFetcher when machine is dead

[luoxn28]

Describe what this PR does / why we need it

in MetricFetche, if machine is dead, CountDownLatch will wait for timeout.

Does this pull request fix one issue?

Describe how you did it

if machine is dead, invoke latch.countDown()

Describe how to verify it

Special notes for reviews

[codecov-io]

Codecov Report

Merging #645 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #645      +/-   ##
============================================
- Coverage     40.49%   40.42%   -0.08%     
+ Complexity     1187     1184       -3     
============================================
  Files           267      267              
  Lines          7845     7845              
  Branches       1061     1061              
============================================
- Hits           3177     3171       -6     
- Misses         4272     4276       +4     
- Partials        396      398       +2

Impacted Files	Coverage Δ	Complexity Δ
...s/block/flow/controller/RateLimiterController.java	79.31% <0%> (-10.35%)	7% <0%> (-1%)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	67.64% <0%> (-2.95%)	33% <0%> (-1%)

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[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!