feat!: struct for metadata for consistency and power

[toddbaert]

This PR alters the new schemas to use maps consistently for all metadata.

Previously, the getMetadata RPC was returning a custom key/val response, which is the old proto2 way of doing maps.

Additionally, for consistency, I have changed the flag metadata to map<string, string> as well. This is a "more" breaking change, but it's consistent and will allow for much stronger typing. If there's a need for more flexibility in flag metadata, we can revert.

We can avoid this change if the breaking is a concern, but I think it's save to assume nobody but us are using this new proto.

[thisthat]

imho the breaking change is fine since flagd still supports the non-namespaced protos and not all languages are still using the new namespaced protos.
if the breaking change is a problem, it is worth considering having this under the v2 ns

[Kavindu-Dodan]

@toddbaert and I discussed this and we concluded that,

• Keeping Struct as internally its typed as map<string, Value> ¹, where Value can be any type ². This allows metadata to contain non-string values, which is useful for future enhancements
• Transfer response type of GetMetadataResponse toStruct for the same advantage (ability to transfer any value type)

Footnotes

1. https://protobuf.dev/reference/protobuf/google.protobuf/#struct ↩

2. https://protobuf.dev/reference/protobuf/google.protobuf/#value ↩

[toddbaert]

@toddbaert and I discussed this and we concluded that,

• Keeping Struct as internally its typed as map<string, Value> 1, where Value can be any type 2. This allows metadata to contain non-string values, which is useful for future enhancements
• Transfer response type of GetMetadataResponse toStruct for the same advantage (ability to transfer any value type)

Footnotes

1. https://protobuf.dev/reference/protobuf/google.protobuf/#struct ↩
2. https://protobuf.dev/reference/protobuf/google.protobuf/#value ↩

Agreed, I've done this. This makes all our metadata consistent. Maybe if we could go back in time I'd use map<string, string>, but Struct also will allow us to do more complex stuff too.

[Kavindu-Dodan]

PR checks / buf-breaking-changes : This is considered as a breaking change, but I think we can proceed anyway

[toddbaert]

@Kavindu-Dodan @thisthat @bacherfl I've marked the old metadata tag (1) as reserved. This is a protective measure for the future, and should reduce the chances of unmarshaling errors.

Only servers using/consuming the old field should be impacted now. See related SO.

[Kavindu-Dodan]

Member

Good improvement. I think this is the way to go to avoid unnecessary issues between flagd & providers.