Proposal for encoding custom metadata :
A specific optional field Id is introduced for custom metadata. Unlike other optional fields (which are defined with a specific data type for the value), the custom field Id is followed by two strings in an MQTT packet. The first string represents a custom name for the optional field and the second string is used for the value.
In an MQTT packet the custom field Id can be repeated, which allows to include multiple custom metadata fields in a single MQTT packet.

I like Konstantin's proposal very much.

I have proposed a way of representing this custom data in MQTT-304 (User Defined CONNECT tags) and if we reach agreement in that Jira, I'll cut and paste the agreed format here too (since it is the same data type, just used in a different command packet)

TC approved on 6/Oct/2016 Resolved, assigned to edit.

Summary:

type name: user defined name-value pair = 0xNN (TBD)

ident : byte = 0xNN

length: Variable Byte Integer
name : UTF-8 encoded string
value : UTF-8 encoded string

The user defined name-value pair is an optional value that used in PUBLISH command packets to carry 'application layer' information which is NOT interpreted by MQTT. This is an optional data item, and may appear zero or more times in a PUBLISH command packet.

A Server receiving this tag MUST check that two string fields contain valid UTF-8 strings. The format, meaning, and uniqueness of the name is not otherwise processed or enforced by MQTT. Servers MUST preserve the order of these tags when processing them.

I thank the TC for the benefit of their comments and prompt consideration.

Added to WD07. Closing.