ExperimentalIdentifier of this message within the enclosing session's transcript.
OptionalsenderOptional sending session identity; omission or null retains a known value.
OptionalrecipientOptional receiving session identity; omission or null retains a known value.
OptionalcontentOmitted leaves content unchanged; null or [] clears it.
A non-empty array replaces all content.
Optional_Omitted leaves metadata unchanged; null removes it.
Implementations MUST NOT make assumptions about values in _meta.
UNSTABLE
This capability is not part of the spec yet, and may be removed or changed at any point.
An upsert for an inter-session message.