feat: add webhook for manually approving traffic weight increase

[mayankshah1607]

What is this change?

This PR adds a new webhook for manually approving traffic weight increase. The logic is similar to confirm-promotion, but for approving traffic weight increase.

Why is this needed?

Organizations have use-cases where they would like to wait indefinitely before the traffic weight on the canary increases. For example, we may want to hold at 10% weight, run some checks, smoke/conformance tests or manually verify metrics to ensure that the canary is reliable. This is especially useful in orgs which are in the process of making metrics and SLOs mature.
(Similar to how argo rollouts works)

Although we could use the rollout webhook for this, it wouldn't wait indefinitely and eventually fail as a non-200 response from the webhook would count as an error.

Example

- name: "confirm traffic increase"
  type: confirm-traffic-increase
  url: http://flagger-loadtester.test/gate/check

Signed-off-by: Mayank Shah [email protected]

[mayankshah1607]

Hey @stefanprodan! Could you take a look at this?
We at Grofers find this feature to be very useful, and would hence really appreciate if this could get merged. At present we're running our own fork that includes this patch.
Thanks :)

[stefanprodan]

@mayankshah1607 can you please change the hook name to confirm-traffic-increase

[stefanprodan]

Can you please add the hook type to docs: https://github.com/fluxcd/flagger/blob/main/docs/gitbook/usage/webhooks.md

[mayankshah1607]

Thanks @stefanprodan , I have made the required changes.

[codecov-io]

Codecov Report

Merging #849 (164bbb8) into main (f2d121a) will decrease coverage by 0.08%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##             main     #849      +/-   ##
==========================================
- Coverage   56.81%   56.72%   -0.09%     
==========================================
  Files          67       67              
  Lines        5418     5430      +12     
==========================================
+ Hits         3078     3080       +2     
- Misses       1881     1890       +9     
- Partials      459      460       +1     

Impacted Files	Coverage Δ
pkg/controller/scheduler_hooks.go	9.67% <11.11%> (+0.24%)	⬆️
pkg/controller/scheduler.go	46.42% <33.33%> (-0.09%)	⬇️

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[stefanprodan]

LGTM

Thanks @mayankshah1607

[mayankshah1607]

@stefanprodan Anything else to be done to get this one merged?

[stefanprodan]

@mayankshah1607 all is Ok, I'll merge it before the next release.

[stefanprodan]

@mayankshah1607 could you add Grofers to the user list here https://github.com/fluxcd/flagger#who-is-using-flagger?