Fix metric name of the 'getpage_wait_seconds_bucket' metric

[hlinnaka]

Per convention, histogram buckets have the '_bucket' suffix. I got that wrong in commit 0d500bb.

Fixes #9241

[tristan957]

I think you meant suffix instead of prefix, but LGTM.

[github-actions]

5022 tests run: 4864 passed, 0 failed, 158 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_gc_of_remote_layers: release-x86-64
• test_secondary_background_downloads: release-x86-64
• test_storage_controller_node_deletion[True]: release-x86-64

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.3% (7485 of 23887 functions)
• lines: 49.6% (60091 of 121239 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c3e07d6 at 2024-10-02T13:51:48.430Z :recycle:}