fix: preinitialize `pageserver_basebackup_query_seconds` metric

[problame]

Without this patch, the Pageserver 4 Golden Signals dashboard shows no data if there are no basebackups (observed in pre-prod).

[github-actions[bot]]

3228 tests run: 3111 passed, 0 failed, 117 skipped (full report)

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Code coverage* (full report)

• functions: 32.4% (6849 of 21152 functions)
• lines: 49.8% (53331 of 107102 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
240fa0db1650096dcbf54e38921ace82fbf7d78c at 2024-06-20T15:49:43.038Z :recycle:}