proxy: Handle all let underscore instances

[cloneable]

• Most can be simply replaced
• One instance renamed to _rtchk (return-type check)

Problem

Summary of changes

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]

3815 tests run: 3709 passed, 0 failed, 106 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_timeline_copy[100000]: release-x86-64

Code coverage* (full report)

• functions: 32.4% (7416 of 22879 functions)
• lines: 50.5% (60027 of 118795 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
939c911 at 2024-09-03T15:54:19.321Z :recycle:}