Fix sts_web_identity credentials provider

[TingDaoK]

Issue #, if available:

• The sts_web_identity double frees the connection to connection manager. And causes a fatal warning and should be an error. But looks like nothing in code actually makes it to fail.

Description of changes:

• Add a count in the mock connection manager to make sure those connection are acquired and released correctly

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.29%. Comparing base (5224ea9) to head (5f24f78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
- Coverage   81.29%   81.29%   -0.01%     
==========================================
  Files          33       33              
  Lines        6250     6248       -2     
==========================================
- Hits         5081     5079       -2     
  Misses       1169     1169              

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