Add event-tags config to targets

[jhujhiti]

These tags are a map whose key/value will be added to event metadata on every subscribe response for the target.

My use-case is to enrich all Prometheus metrics with a "site" label. For example, the config

targets:
  switch1:
    event-tags:
      site: sitea
  switch2:
    event-tags:
      site: siteb

will result in events and ultimately Prometheus metrics (in my case) with an extra label site=sitea/siteb.

I would have preferred to call the field simply tags, but it's already in use by the clustering function and I didn't want to overload its meaning.

[karimra]

Thanks for this contribution, although the same result can be achieved using the event-add-tag processor, this way is more simple for static tags per target.

[jhujhiti]

I had tried to use event-add-tag to accomplish it, actually, but in my case, I had no value to pull out of an existing tag (like source) nor any consistent way to match another tag or value to identify unique groupings to assign a static tag to. Basically, the one I was trying to add was unknowable based on the gNMI stream or existing metadata.