Support raw messages metadata per component in PubSub

In what area(s)?

/area runtime

/area operator /area placement /area docs /area test-and-release

Describe the feature

It would like to propose that we add metadata support in the PubSub API. If I understand things correctly, the API presently provides a mechanism to send a binary package over a topic. As an example, let's take a scenario where a topic could broker different types of messages using different serialization protocols. In this case:

• The client assumes type and serialization mechanism of the event (inflexible)
• The client performs a binary inspection of the payload to infer type and serialization mechanism (expensive)

In this case, the payload could also carry some kind of text "content-type" header that would simplify the life of the consumer.

Metadata would simply be a collection of string k,v couples. Nothing more complicated than that.

Release Note

RELEASE NOTE: