Strict YAML validation rejects `custom_`-prefixed metadata fields in project files

[avaitla]

Problem

Teams adding custom fields like an owners list in yaml or building on top of Rill (custom distributions) need to attach user-defined metadata to project files so their tooling can key off it. Today, adding any unrecognized field to a strictly validated resource (metrics views, explores, canvases, alerts, reports, components, rill.yaml) fails parsing:

yaml: unmarshal errors: line 7: field custom_field not found in type parser.MetricsViewYAML

This happens at the top level and in nested fields (e.g. inside a dimension or measure), because yaml.v3's KnownFields(true) applies recursively to the whole document. As a result, a project containing such metadata cannot be deployed on the stock Rill product.

Proposal

Exempt any YAML key prefixed with custom_ from strict validation, at any nesting depth:

• Stock Rill accepts and ignores custom_-prefixed keys, so the same project files deploy cleanly on Rill Cloud.
• Custom distributions that define matching custom_-prefixed fields on their parser structs still receive the values.
• All other unknown fields keep erroring, so typo detection is preserved.
• The prefix is easy to grep for and easy to comment in and out.

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Developed in collaboration with Claude Code