@type t() :: %Envoy.Extensions.Filters.Http.ThriftToMetadata.V3.KeyValuePair{
__unknown_fields__: [
{field_number :: integer(), Protobuf.Wire.Types.wire_type(),
value :: term()}
],
key: String.t(),
metadata_namespace: String.t(),
value: Google.Protobuf.Value.t() | nil
}