pageserver config: ignore unknown fields (instead of deny_unknown_fields)

[problame]

Refs

• refs remote_storage_config deserialization does not reject unknown fields #8915
• discussion thread: https://neondb.slack.com/archives/C033RQ5SPDH/p1742406381132599
• stacked atop impr: merge pageserver_api::models::TenantConfig and pageserver::tenant::config::TenantConfOpt #11298
• corresponding internal docs update: https://github.com/neondatabase/docs/pull/404

Problem

Rejecting pageserver.tomls with unknown fields adds friction, especially when using pageserver.toml fields as feature flags that need to be decommissioned.

See the added paragraphs on pageserver_api::models::ConfigToml for details on what kind of friction it causes.

Also reed the corresponding internal docs update linked above to see a more imperative guide for using pageserver.toml flags as feature flags.

Solution

Remove serde(deny_unknown_fields) from all structs in pageserver_api::config::ConfigToml pageserver_api::config::TenantConfigToml.

I went through all the child fields and verified they don't use deny_unknown_fields either, including those shared with pageserver_api::models.

TODO

• pageserver config: warn about unknown config items during startup #11322

Future Work

Tooling to detect usage of unknown fields:

• https://github.com/neondatabase/cloud/issues/24349

[github-actions]

7964 tests run: 7580 passed, 0 failed, 384 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_scrubber_tenant_snapshot[4]: debug-x86-64-without-lfc
• test_timeline_archival_chaos: release-arm64-without-lfc

Postgres 14

• test_replica_query_race: release-x86-64-with-lfc

Code coverage* (full report)

• functions: 32.4% (8722 of 26950 functions)
• lines: 48.4% (74842 of 154539 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
13fa4b4 at 2025-03-19T19:14:14.763Z :recycle:}

[erikgrinaker]

LGTM, but let's add warnings for unknown fields too, as discussed elsewhere.