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_test.go

@@ -0,0 +1,33 @@
+package duration_test
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/xtls/xray-core/infra/conf/cfgcommon/duration"
+)
+
+type testWithDuration struct {
+	Duration duration.Duration
+}
+
+func TestDurationJSON(t *testing.T) {
+	expected := &testWithDuration{
+		Duration: duration.Duration(time.Hour),
+	}
+	data, err := json.Marshal(expected)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	actual := &testWithDuration{}
+	err = json.Unmarshal(data, &actual)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	if actual.Duration != expected.Duration {
+		t.Errorf("expected: %s, actual: %s", time.Duration(expected.Duration), time.Duration(actual.Duration))
+	}
+}