Add custom JSON marshaller for Alertmanager configurations (#349)

* Add a custom JSON marshaller for Alertmanager configurations

* Apply suggestions from code review

---------

Co-authored-by: Fayzal Ghantiwala <114010985+fayzal-g@users.noreply.github.com>

Author: alexander-akhmetov

definition/json.go

@@ -0,0 +1,85 @@
+package definition
+
+import (
+	"unsafe"
+
+	jsoniter "github.com/json-iterator/go"
+	"github.com/modern-go/reflect2"
+	amcfg "github.com/prometheus/alertmanager/config"
+	commoncfg "github.com/prometheus/common/config"
+)
+
+// secretEncoder encodes Secret to plain text JSON,
+// avoiding the default masking behavior of the structure.
+type secretEncoder struct{}
+
+func (encoder *secretEncoder) Encode(ptr unsafe.Pointer, stream *jsoniter.Stream) {
+	stream.WriteString(getStr(ptr))
+}
+
+func (encoder *secretEncoder) IsEmpty(ptr unsafe.Pointer) bool {
+	return len(getStr(ptr)) == 0
+}
+
+func getStr(ptr unsafe.Pointer) string {
+	return *(*string)(ptr)
+}
+
+// secretEncoder encodes SecretURL to plain text JSON,
+// avoiding the default masking behavior of the structure.
+type secretURLEncoder struct{}
+
+func (encoder *secretURLEncoder) Encode(ptr unsafe.Pointer, stream *jsoniter.Stream) {
+	url := getURL(ptr)
+	if url.URL != nil {
+		stream.WriteString(url.String())
+	} else {
+		stream.WriteNil()
+	}
+}
+
+func (encoder *secretURLEncoder) IsEmpty(ptr unsafe.Pointer) bool {
+	url := getURL(ptr)
+	return url.URL == nil
+}
+
+func getURL(ptr unsafe.Pointer) *amcfg.URL {
+	v := (*amcfg.SecretURL)(ptr)
+	url := amcfg.URL(*v)
+	return &url
+}
+
+func newPlainAPI() jsoniter.API {
+	api := jsoniter.ConfigCompatibleWithStandardLibrary
+
+	secretEnc := &secretEncoder{}
+	secretURLEnc := &secretURLEncoder{}
+
+	extension := jsoniter.EncoderExtension{
+		// Value types
+		reflect2.TypeOfPtr((*amcfg.Secret)(nil)).Elem():     secretEnc,
+		reflect2.TypeOfPtr((*commoncfg.Secret)(nil)).Elem(): secretEnc,
+		reflect2.TypeOfPtr((*amcfg.SecretURL)(nil)).Elem():  secretURLEnc,
+		// Pointer types
+		reflect2.TypeOfPtr((*amcfg.Secret)(nil)):     &jsoniter.OptionalEncoder{ValueEncoder: secretEnc},
+		reflect2.TypeOfPtr((*commoncfg.Secret)(nil)): &jsoniter.OptionalEncoder{ValueEncoder: secretEnc},
+		reflect2.TypeOfPtr((*amcfg.SecretURL)(nil)):  &jsoniter.OptionalEncoder{ValueEncoder: secretURLEnc},
+	}
+
+	api.RegisterExtension(extension)
+
+	return api
+}
+
+var (
+	plainJSON = newPlainAPI()
+)
+
+// MarshalJSONWithSecrets marshals the given value to JSON with secrets in plain text.
+//
+// alertmanager's and prometheus' Secret and SecretURL types mask their values
+// when marshaled with the standard JSON or YAML marshallers. This function
+// preserves the values of these types by using a custom JSON encoder.
+func MarshalJSONWithSecrets(v any) ([]byte, error) {
+	return plainJSON.Marshal(v)
+}