Implement telegraf collecting stats on itself

closes #1348

selfstat/stat.go

@@ -0,0 +1,50 @@
+package selfstat
+
+import (
+	"sync/atomic"
+)
+
+type stat struct {
+	v           int64
+	measurement string
+	field       string
+	tags        map[string]string
+	key         uint64
+}
+
+func (s *stat) Incr(v int64) {
+	atomic.AddInt64(&s.v, v)
+}
+
+func (s *stat) Set(v int64) {
+	atomic.StoreInt64(&s.v, v)
+}
+
+func (s *stat) Get() int64 {
+	return atomic.LoadInt64(&s.v)
+}
+
+func (s *stat) Name() string {
+	return s.measurement
+}
+
+func (s *stat) FieldName() string {
+	return s.field
+}
+
+// Tags returns a copy of the stat's tags.
+// NOTE this allocates a new map every time it is called.
+func (s *stat) Tags() map[string]string {
+	m := make(map[string]string, len(s.tags))
+	for k, v := range s.tags {
+		m[k] = v
+	}
+	return m
+}
+
+func (s *stat) Key() uint64 {
+	if s.key == 0 {
+		s.key = key(s.measurement, s.tags)
+	}
+	return s.key
+}