Fix timer not released

[hailin0]

Fix

• Add a unit test to verify that the fix works.
• Explain briefly why the bug exists and how to fix it.

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes https://github.com/apache/skywalking/issues/.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #300 (d78730e) into main (df13bd6) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             main     #300   +/-   ##
=======================================
  Coverage   44.22%   44.23%           
=======================================
  Files          93       93           
  Lines        9811     9814    +3     
=======================================
+ Hits         4339     4341    +2     
- Misses       5054     5055    +1     
  Partials      418      418           

Impacted Files	Coverage Δ
pkg/index/inverted/inverted.go	69.72% <66.66%> (-0.03%)	⬇️

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