Parity between OperationResultCode and CompletableResultCode for deferred results

[fractalwrench]

Originally raised in the old repository: embrace-io/opentelemetry-kotlin#445

There seems to be a notable gap in the expressiveness of OperationResultCode compared to OTel Java's CompletableResultCode, particularly around accommodation of deferred results. Curious to hear from the maintainers on whether there is a plan to address this parity.

One place where this becomes apparent is in TelemetryExporter#export(List<T>) as Success is immediately returned regardless of whether there is a downstream failure. Retrial mechanism notwithstanding, this could provide a false impression on the state of export to the client.