Single controller deployments: Admin API endpoints tracking via EndpointSlice controller

[pmalek]

Is there an existing issue for this?

• I have searched the existing issues

Problem Statement

With #3268 we're able to specify multiple Admin API endpoints using --kong-admin-url CLI flag.

This issue treats about the implementation of Kong Admin API service discovery so that its (Kong Gateway's Admin API) addresses are updated in real time during controller's runtime.

Proposed Solution

Implement service discovery through the means of Endpoints controller that will track Admin API Service's endpoints and pass those to synchronizer.Synchronizer (or one of its subcomponents if that ends up a better approach).

The name of Kong Admin API (headless) service will be provided via a new flag - proposal --kong-admin-svc to align with existing --kong-admin-url that serves a similar purpose - that will be mutually exclusive with --kong-admin-url.

Additional information

No response

Acceptance Criteria

• As a user I can specify a Kong Admin API Service name through means of CLI flag and environment variable (but not at the same time) which will be tracked for Service endpoints which will be used to configure Kong Gateway connected with those endpoints
• When working with horizontally scaled KIC with more than 1 replica, leader election works and only 1 KIC sends configuration payloads to Kong Gateways #3528