Follow-up from "Filter Geo proxied data from api_json.log"

The following discussion from gitlab-ce!30687 should be addressed:

• @ashmckenzie started a discussion: (+2 comments)

  I agree we should filter out the output but I feel filtering on /^output$/ could (either now or in the future) potentially swallow up some other valuable output. Alternatives I'm thinking about:

  1. I don't know if it's possible, but if we could filter out output before config.filter_parameters is consulted that feels like a more accurate solution
  2. Change the output key to something more specific like encoded_git_payload and then filter out ^encoded_git_payload$/

  If 1. isn't possible (and regardless of 2.), we'd also need to ensure we create a workhorse MR to appease:

  # NOTE: It is **IMPORTANT** to also update gitlab-workhorse's filter when adding parameters here to not
  #       introduce another security vulnerability: https://gitlab.com/gitlab-org/gitlab-workhorse/issues/182

  Thoughts?

Let's implement option 2. in this Issue, which will mean changes required to gitlab and gitlab-shell with careful consideration to allow supporting the current output param name and the new name.