Confmap does not honor Unmarshal methods on embedded structs

[mx-psi]

Describe the bug

The confmap Unmarshal method honors the Unmarshal methods on field structs but does not do so on embedded structs.

Steps to reproduce

See #6668 for an example

What version did you use?

Happens as of fb0842a

[mx-psi]

I spent a bit working on this but I am not sure how to fix it. It looks related to mitchellh/mapstructure/issues/166, so maybe @jefchien has some ideas?

[jefchien]

This seems like a different issue. When mapstructure tries to decode a map into a struct, it has this loop that handles the squashing of the embedded struct fields https://github.com/mitchellh/mapstructure/blob/main/mapstructure.go#L1340. Since it happens in this step, it doesn't try to decode the embedded struct as a whole and only calls decode on the fields within. So, the decode hook, which is used to assert/call the Unmarshaler interface (https://github.com/open-telemetry/opentelemetry-collector/blob/main/confmap/confmap.go#L271) never sees the embedded struct.

[mx-psi]

I checked this with @gbbr and we don't see a way to fix it short of re-implementing the Decode method itself (which means a few thousand lines to maintain). I am tempted to say we mark this as 'wontfix' and just document this limitation clearly: it's annoying and unexpected, but there is a workaround.

[bogdandrutu]

In the current unmarshalerHookFunc can we iterate over all the fields of the "to" and if a struct field (without name) then we check for unmarshaler... Does that miss something?