Reconciler needs to provide a filter function for leader election

Expected Behavior

Tekton has created a new custom resource called Run for running custom tasks. Each custom task implementer will provide a reconciler that watches for and processes Run objects that refer to its kind of custom task, through the use of the filter function given to the run informer.

Actual Behavior

We've found a case where a reconciler is given all Run objects, not just the ones it has requested through the filter function given to the run informer. This happens during leader election in this code:

https://github.com/tektoncd/pipeline/blob/master/pkg/client/injection/reconciler/pipeline/v1alpha1/run/controller.go#L72

A custom task implementer will not want to be given Run objects that it isn't written to handle. It needs to be able to pass a filter function to filter the objects on this path.

Steps to Reproduce the Problem

Any controller using leader election goes through this path. Run objects may be the first case though where there may be multiple independent controllers that are only interested in distinct subsets of the objects.

Additional Info

Start at pkg/client/injection/reconciler/pipeline/v1alpha1/run/controller.go around line 72 and trace the leader-election path and the filter function passed to the Run informer. Done means controllers using leader election preserve the requested filter so custom-task reconcilers receive only the Run objects they handle; verify against the reproduction described in the issue.