Snowplow server-side events should respect DNT: 1 header

The following discussion from !19463 (merged) should be addressed:

• @nick.thomas started a discussion: (+1 comment)

  Again, we can merge this MR without fixing this here, and address it in a follow-up issue, but I didn't want to lose track of it.

  Our snowplow implementation is supposed to honour DNT: 1 in requests, so surely this code (and other code like it across the codebase) should be skipped over if that's present in the request? We respect DNT: 1 for events gathered client-side, and I think it's appropriate that we do the same for events gathered server-side.