Add observatory / latestPing balancing strategy

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>

app/observatory/explainErrors.go

@@ -0,0 +1,26 @@
+package observatory
+
+import "github.com/xtls/xray-core/common/errors"
+
+type errorCollector struct {
+	errors *errors.Error
+}
+
+func (e *errorCollector) SubmitError(err error) {
+	if e.errors == nil {
+		e.errors = newError("underlying connection error").Base(err)
+		return
+	}
+	e.errors = e.errors.Base(newError("underlying connection error").Base(err))
+}
+
+func newErrorCollector() *errorCollector {
+	return &errorCollector{}
+}
+
+func (e *errorCollector) UnderlyingError() error {
+	if e.errors == nil {
+		return newError("failed to produce report")
+	}
+	return e.errors
+}