Fix double escaping in the metadata of FORMAT JSON

src/Formats/JSONUtils.cpp

@@ -529,7 +529,12 @@ namespace JSONUtils
             writeObjectStart(out, 2);
 
             writeTitle<true>("name", out, 3);
-            writeDoubleQuoted(fields[i].name, out);
+
+            /// The field names are pre-escaped to be put into JSON string literal.
+            writeChar('"', out);
+            writeString(fields[i].name, out);
+            writeChar('"', out);
+
             writeFieldDelimiter(out);
             writeTitle<true>("type", out, 3);
             writeJSONString(fields[i].type->getName(), out, settings);

src/Processors/Formats/Impl/JSONRowOutputFormat.h

@@ -72,7 +72,7 @@ protected:
 
     size_t field_number = 0;
     size_t row_count = 0;
-    NamesAndTypes fields;
+    NamesAndTypes fields;   /// The field names are pre-escaped to be put into JSON string literal.
 
     Statistics statistics;
     FormatSettings settings;

tests/queries/0_stateless/02375_double_escaping_json.reference

@@ -0,0 +1,18 @@
+{
+	"meta":
+	[
+		{
+			"name": "\"",
+			"type": "String"
+		}
+	],
+
+	"data":
+	[
+		{
+			"\"": "\\"
+		}
+	],
+
+	"rows": 1
+}

tests/queries/0_stateless/02375_double_escaping_json.sql

@@ -0,0 +1 @@
+SELECT '\\' AS `"` FORMAT JSON SETTINGS output_format_write_statistics = 0;