plugin/pkg/uniq: add (#1733)

Spin this out the metrics package so we can use it in the health
one of well to fix some reload bugs.

plugin/pkg/uniq/uniq.go

@@ -0,0 +1,52 @@
+// Package uniq keeps track of "thing" that are either "todo" or "done". Multiple
+// identical events will only be processed once.
+package uniq
+
+// U keeps track of item to be done.
+type U struct {
+	u map[string]item
+}
+
+type item struct {
+	state int          // either todo or done
+	f     func() error // function to be executed.
+}
+
+// New returns a new initialized U.
+func New() U { return U{u: make(map[string]item)} }
+
+// Set sets function f in U under key. If the key already exists
+// it is not overwritten.
+func (u U) Set(key string, f func() error) {
+	if _, ok := u.u[key]; ok {
+		return
+	}
+	u.u[key] = item{todo, f}
+}
+
+// SetTodo sets key to 'todo' again.
+func (u U) SetTodo(key string) {
+	v, ok := u.u[key]
+	if !ok {
+		return
+	}
+	v.state = todo
+	u.u[key] = v
+}
+
+// ForEach iterates for u executes f for each element that is 'todo' and sets it to 'done'.
+func (u U) ForEach() error {
+	for k, v := range u.u {
+		if v.state == todo {
+			v.f()
+		}
+		v.state = done
+		u.u[k] = v
+	}
+	return nil
+}
+
+const (
+	todo = 1
+	done = 2
+)