config: add jitter interval for watcher to avoid dos

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

util/jitter/random.go

@@ -0,0 +1,15 @@
+// Package jitter provides a random jitter
+package jitter
+
+import (
+	"time"
+
+	"github.com/unistack-org/micro/v3/util/rand"
+)
+
+// Random returns a random time to jitter with max cap specified
+func Random(d time.Duration) time.Duration {
+	var rng rand.Rand
+	v := rng.Float64() * float64(d.Nanoseconds())
+	return time.Duration(v)
+}