newly tagged subnets are not added to load balancer via subnet discovery

[nwsparks]

Is your feature request related to a problem?

Past issues:

#2930
#3157

If a tag is added to a new subnet the LB controller does not add it to the existing load balancer. This is also not reflected in the documentation so when the problem occurs its rather difficult to track down.

Describe the solution you'd like

ideal: aws-lb-controller should check for subnet updates and reconcile existing load balancers. The default action could be similar to the approach taken by external-dns's upsert-only mode and only add new subnets to the lb's vs removing them if tags are removed.

cope: update the docs to reflect this behavior