Observatory related fixes (#788)

* fix:observatory not supported by multi-json

* Fix: observatory starts with empty config & fails to close (#957)

* Update strategy_leastping.go (#1019)

* add custom probe URL support for observatory

* add custom probe interval for observer

* apply coding style

* Fix: observatory log & JSON config(#1211)

Co-authored-by: ihotte <ihotte@yeah.net>

* Change default probe url from api.v2fly.org to www.google.com

* Cherry-pick missing code from branch 'dev-advloadblancer-2'

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>
Co-authored-by: Loyalsoldier <10487845+Loyalsoldier@users.noreply.github.com>
Co-authored-by: fanyiguan <52657276+fanyiguang@users.noreply.github.com>
Co-authored-by: ihotte <3087168217@qq.com>
Co-authored-by: ihotte <ihotte@yeah.net>

infra/conf/cfgcommon/duration/duration.go

@@ -0,0 +1,33 @@
+package duration
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+type Duration int64
+
+func (d *Duration) MarshalJSON() ([]byte, error) {
+	dr := time.Duration(*d)
+	return json.Marshal(dr.String())
+}
+
+func (d *Duration) UnmarshalJSON(b []byte) error {
+	var v interface{}
+	if err := json.Unmarshal(b, &v); err != nil {
+		return err
+	}
+	switch value := v.(type) {
+	case string:
+		var err error
+		dr, err := time.ParseDuration(value)
+		if err != nil {
+			return err
+		}
+		*d = Duration(dr)
+		return nil
+	default:
+		return fmt.Errorf("invalid duration: %v", v)
+	}
+}