A key-value pair included in the HTTP header. It is a 2-tuple where the first entry is the key and the second is the value, i.e. (key, value). The metadata key is an ASCII str, and must be a valid HTTP header name. The metadata value can be either a valid HTTP ASCII str, or bytes. If bytes are provided, the key must end with ‘-bin’, i.e. ('binary-metadata-bin', b'\\x00\\xFF')
A sequence of metadatum.