backend: cmd/pagination: add pagination API for getting limited resource size

[upsaurav12]

This add new pagination API which would help to send only request that the user is veiwing only not just full resource list.

How To Test.

Currently this functionality is not integrated with the frontend of Headlamp but still we can see the pagination works easily if we have tools like POSTMAN or ThunderClient (VS Code Extension).

1. Make sure that you have resource that have list over 1000's of items or even 100 would be fine.
2. Then open POSTMAN and add something like this localhost:4466/clusters/{your-cluster-name}/api/v1/{your-desired-resource}?pageNo=<page-you-want-to-go>.
3. Enter it now you should see only page that you requested (15 items which pageNo).
4. Also see the response timing you can see how much time is it taking to paginate the items, from my POV it is taking around 30ms for 1000's of secrets,
5. Also you can see how much KB does client is taking, from my POV for 1000's of secrets normally it is taking 74KB which means it is downloading full items list, but with pagination it is downloading only 1.12KB which means it is downloading only the page which was requested.
6. You can also test using load-tests and create 1000's of pods and nodes, there you can see that the client is downloading the resource page of that page with size of ~2KB, and with the current setup it is downloading 100KB.

here is my current implementation for pagination, i am currently figuring out how i am going to integrate this with frontend so that we can see the actual change due to pagination.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: upsaurav12
Once this PR has been reviewed and has the lgtm label, please assign joaquimrocha for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[upsaurav12]

@yolossn @illume I have tested this with POSTMAN client tool where i have to add continueToken for the next batch of resource list which means it is working as expected.

only thing we would need is to set continueToken in frontend .

[upsaurav12]

@illume @yolossn the only problem in this approach would be that if there are > 1000's of pods/nodes or other heavy resources it would take 800ms for returning that paginating data.

[upsaurav12]

@illume @yolossn I am trying to implement the pagination part but here the problem is the it is somehow still taking 800ms or even 1s taking from the backend server. I checked in postman it is giving response time like 10ms etc.

Here in this code i am adding query that will go to backend server which results adding p unneccesarily into URL.
for example if i am on secrets page it should add /api/v1/secrets?p=1,2,3,4 etc but it is also adding /api/v1/namespace=5,6,7 even mutating pageNo with many resources here is the screenshot from network tab, it seems backend is working fine but rather frontend is doing more work

I think i need to manage the pageState in such a way that it should send pageNo to backend when the user is going to next or prev page

[k8s-ci-robot]

PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.