proxy: fix credentials cache lookup

[khanova]

Problem

Incorrect processing of -pooler connections.

Summary of changes

Fix

TODO: add e2e tests for caching

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2772 tests run: 2654 passed, 0 failed, 118 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_partial_evict_tenant[relative_equal]: release

Code coverage* (full report)

• functions: 28.0% (6434 of 22955 functions)
• lines: 46.7% (45173 of 96661 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6315cc1 at 2024-04-10T08:44:53.154Z :recycle:}