feat: slow threshold customizable in redis (#1185)

* feat: slow threshold customizable in redis

* chore: improve config robustness

core/conf/time.go

@@ -0,0 +1,16 @@
+package conf
+
+import "time"
+
+const minDuration = 100 * time.Microsecond
+
+// CheckedDuration returns the duration that guaranteed to be greater than 100us.
+// Why we need this is because users sometimes intend to use 500 to represent 500ms.
+// In config, duration less than 100us should always be missing ms etc.
+func CheckedDuration(duration time.Duration) time.Duration {
+	if duration > minDuration {
+		return duration
+	}
+
+	return duration * time.Millisecond
+}