Add ADR for ServiceLB move to CCM

docs/adrs/servicelb-ccm.md

+# Move the ServiceLB load-balancer controller into the K3s cloud provider
+
+Date: 2022-09-29
+
+## Status
+
+Accepted
+
+## Context
+
+K3s includes a stub cloud-provider the implements just enough node lifecycle functionality (the
+`cloudprovider.Instances` interface) to get node addresses set properly, and clear the Uninitialized taint
+that is added to nodes when they first join the cluster. The cloud-provider interface also has extension
+points for load-balancer controllers, but we did not implement these, in favor of running a standalone
+ServiceLB controller that is directly hooked into the core Wrangler controllers.
+
+Because it doesn't make use of the existing load-balancer interface, the ServiceLB controller must implement
+all the logic to watch Services, ensure that it's only handling services of the correct type and state, manage
+finalizers, and so on.  This would all be handled by core Kubernetes code if we implemented the
+`cloudprovider.LoadBalancer` interface.
+
+## Decision
+
+We will move the ServiceLB code into the cloud-controller, as a backend for the LoadBalancer interface
+implementation. Existing behavior for disabling node lifecycle functionality will be retained, such that users
+can still use ServiceLB alongside other cloud-controller-managers that handle node lifecycle. Support for
+customizing ServiceLB behavior via node labels will be retained.
+
+## Consequences
+
+* K3s uses less resources when ServiceLB is disabled, as several core controllers are no longer started
+unconditionally.
+* The `--disable-cloud-controller` flag now disables the CCM's `cloud-node` and `cloud-node-lifecycle`
+controllers that were historically the only supported controllers.
+* The `--disable=servicelb` flag now disables the CCM's `service` controller.
+* If the cloud-controller and servicelb are both disabled, the cloud-controller-manager is not run at all.