feat: support simple selection of labels in json expr parser (#6151)

docs/sources/logql/log_queries.md

@@ -364,6 +364,18 @@ The **json** parser operates in two modes:
    "server_list" => `["129.0.1.1","10.2.1.3"]`
    "headers" => `{"Accept": "*/*", "User-Agent": "curl/7.68.0"}`
    ```
+ 
+   If the label to be extracted is same as the original JSON field, expression can be written as just `| json <label>`
+
+   For example, to extract `servers` fields as label, expression can be written as following
+    
+   `| json servers` will extract:
+
+    ```kv
+   "servers" => `["129.0.1.1","10.2.1.3"]`
+   ```
+
+   Note that `| json servers` is same as `| json servers="servers"`
 
 #### logfmt
 

pkg/logql/syntax/expr.y

@@ -301,6 +301,7 @@ labelFilter:
 
 jsonExpression:
     IDENTIFIER EQ STRING { $$ = log.NewJSONExpr($1, $3) }
+  | IDENTIFIER { $$ = log.NewJSONExpr($1, $1) }
 
 jsonExpressionList:
     jsonExpression                          { $$ = []log.JSONExpression{$1} }

pkg/logql/syntax/parser_test.go

@@ -2943,6 +2943,18 @@ func TestParse(t *testing.T) {
 				},
 			},
 		},
+		{
+			in: `{app="foo"} | json response_code, api_key="request.headers[\"X-API-KEY\"]"`,
+			exp: &PipelineExpr{
+				Left: newMatcherExpr([]*labels.Matcher{{Type: labels.MatchEqual, Name: "app", Value: "foo"}}),
+				MultiStages: MultiStageExpr{
+					newJSONExpressionParser([]log.JSONExpression{
+						log.NewJSONExpr("response_code", `response_code`),
+						log.NewJSONExpr("api_key", `request.headers["X-API-KEY"]`),
+					}),
+				},
+			},
+		},
 	} {
 		t.Run(tc.in, func(t *testing.T) {
 			ast, err := ParseExpr(tc.in)