Add attributes option to GCP PubSub output

[annadorottya]

Motivation

We would like to be able to add attributes to the PubSub messages so we can categorize the incoming messages from different sources to the topic.

Feature

A list of key-value pairs in the config below the pubsub option, which will be added to all the output messages as attributes.

Alternatives

Custom fields are not a good substitution since we can only filter messages by their attributes, but not by the data in the message. source

Additional context