osbase/supervisor: implement Prometheus metrics

This is a simple implementation of supervisor.Metrics which will be used
in Metropolis.

Change-Id: I8d47b3aa631dec5b07295d5498b3b0d1ad36c0f7
Reviewed-on: https://review.monogon.dev/c/monogon/+/3291
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
Tested-by: Jenkins CI
diff --git a/osbase/supervisor/supervisor_node.go b/osbase/supervisor/supervisor_node.go
index 44e8c84..272b650 100644
--- a/osbase/supervisor/supervisor_node.go
+++ b/osbase/supervisor/supervisor_node.go
@@ -84,6 +84,15 @@
 	NodeStateCanceled
 )
 
+// NodeStates is a list of all possible values of a NodeState.
+var NodeStates = []NodeState{
+	NodeStateNew,
+	NodeStateHealthy,
+	NodeStateDead,
+	NodeStateDone,
+	NodeStateCanceled,
+}
+
 func (s NodeState) String() string {
 	switch s {
 	case NodeStateNew: