Fix query error in vm-image-spec.yaml

[MMeent]

This query causes metrics exporter to complain about missing data because it can't find the correct column.

Issue was introduced with #7761

Problem

There are a lot of logs Error gathering metrics: [from Gatherer #1] [collector=neon_collector,query=logical_slot_restart_lsn] Missing values for the requested columns: ["restart_lsn"]

Summary of changes

This fixes the query so that it does have an output column restart_lsn

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]

3210 tests run: 3068 passed, 0 failed, 142 skipped (full report)

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Flaky tests (2)

Postgres 16

• test_vm_bit_clear_on_heap_lock_blackbox: debug

Postgres 15

• test_pageserver_restarts_under_worload: release

Code coverage* (full report)

• functions: 31.6% (6634 of 20990 functions)
• lines: 48.6% (51529 of 106045 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
da19c62 at 2024-06-12T12:02:40.686Z :recycle:}