Add global_tags annotation

[mkmik]

Oftentimes you don't need a brand new class. All you want is to add a common tag to all the metrics emitted by a given pod.

Proposal

annotations:
  telegraf.influxdata.com/global-tag-literal-region: us-east1
  telegraf.influxdata.com/global-tag-fieldref-ns: metadata.namespace
  telegraf.influxdata.com/global-tag-configmapkeyref-foo: bar.baz

equivalent to use this generated config:

    [global_tags]
      region = "us-east1"
      ns = "myns"
      foo = "quz"

(the implementation may choose to use some intermediate env vars if so desired)

[mkmik]

from a quick peek in the sources I noticed that technically the telegraf.influxdata.com/inputs annotation can be abused to inject a [global_tags] snippet, however this doesn't work well when the "class" configs already contain a the global_tags.

A first class feature telegraf.influxdata.com/global-tag-... feature should be able to merge the global tags from the "system" config with the tags defined by the pod annotation.

[mkmik]

(@cbos I see you did something similar in spirit in in #75, perhaps you'll find this useful)

[cbos]

@mkmik Thanks for the notification, this is indeed a useful feature.