Consider parsing/rejecting items in containers individually

[loewenheim]

Relay sometimes receives item containers containing many items (e.g. logs) of which most are OK but some are unparseable. Currently it rejects the entire container in such a situation. It would make more sense to parse and discard items individually.

ref: getsentry/sentry-react-native#5186

[linear-code]

RELAY-219

[knQzx]

just looked at this and the linked sentry-react-native#5186 - happy to take it on if nobody's actively on it

two things i'd want to confirm before digging in:

• is the scope logs-container only for now (since that's what the RN issue surfaces), or do you want a generic per-item parsing path covering every item-container variant up front
• on rejection: should bad items get their own outcome (invalid/filtered) per item, or is structured logging enough for v1

if there's a preferred direction i'll just go with it, otherwise i can put up a draft against logs first to nail the shape and we can generalise from there

[Dav1dde]

Scope wise it should work for all item containers, this is fundamentally useful.

My rough idea: The generic container now not only returns items which were parsed successfully but also items which failed to parse. I am not yet clear how this would look, it could be Vec<Result<T, E>> or {values: Vec<T>, error: Vec<Error> or just an informational count (values: Vec<T>, errors: usize) of how many items failed to parse. Latter is probably the easiest to deal with and for now we don't have a usecase for keeping track of individual errors -> probably a good starting point.

Implementation wise, something like TryMetadata could be a way to do it, a custom deserializer for Vec<T> which also keeps track of failures.

For your second question, bad items should get an informational outcome, Outcome::Invalid(DiscardReason::InvalidJson) seems appropriate. But feel free to just keep the scope clean and start with the item container parts.

Fair warning, I haven't put too much thought into this, there is a chance things won't work out the way I outlined them here.

Thanks for taking a look!