Why a window of async meter events was rejected: a total error count, and the failures grouped by error code.

Stripe already did the grouping and the counting

error_count appears twice in this payload — once here, as the total across the whole validation window, and once on each LatticeStripe.Billing.MeterErrorReport.ErrorType as that code's own count. Stripe supplies both, and it supplies the grouping itself via error_types.

That is why this library ships zero grouping or counting helpers on the error report. An Enum.group_by/2 wrapper would re-derive a grouping the wire already handed you, and a count helper would sum a number Stripe already sent — both would be slower and both could disagree with Stripe. The wire supplies the ergonomics.

Note the two counts can legitimately differ from what the samples show: see LatticeStripe.Billing.MeterErrorReport.ErrorType on why sample_errors is a sample rather than a complete list.

Nullability

reason is nullable on the wire, so from_map/1 returns nil for nil. error_types, when present at all, decodes to a list of typed structs; when absent it decodes to [].