Add a separate rollback check endpoint

[harrison-kim]

We currently have the following within our canary manifest:

      # The /gate/check endpoint returns 403 until /gate/open is called
      - name: "promotion gate"
        type: confirm-promotion
        url: http://flagger-loadtester.test/gate/check
      # Close the gate after rollout to block future deployments
      - name: "close gate"
        type: post-rollout
        url: http://flagger-loadtester.test/gate/close

This works out great for halting promotions and requiring manual approval within our pipelines!

However, rollbacks have been a bit more difficult. We wanted to have rollback follow a similar procedure without having to manually edit the manifest from /halt to an endpoint that returns a 200, as provided within the examples:

      - name: "rollback"
        type: rollback
        url: http://flagger-loadtester.test/gate/halt

Is it possible to create a separate /rollback/check, rollback/open, and rollback/close endpoint for this purpose? The current alternative is to spin up a second loadtester deployment (e.g. flagger-loadtester-rollback).

[ta924]

@stefanprodan if you have recommendations on how you envision this approach working, I can submit a PR to handle rollback gating. I could think of a few options, the first to continue with the approach now for GateStorage and add RollbackStorage. Second, modify the GateStorage map to handle keys specifying specific gating approaches. Exposing the endpoints is the simple part, so the underlying storage seems to be the only real question to resolve.

[stefanprodan]

@ta924 I would add rollback as a prefix to the GateStorage keys and create the /rollback/[check,open,close] routers.

[ta924]

Thanks @stefanprodan for the feedback, I will get a PR out there.

[stefanprodan]

Fixed by #449