Add more tests for the ndjson parser behavior (#47774)

When parsing JSON objects with duplicated keys it was not clear how our parser behaves.
These tests are to clarify the behavior which should help with the future debugging efforts.

Author: rdner

libbeat/reader/readjson/json_test.go

@@ -89,6 +89,16 @@ func TestUnmarshal(t *testing.T) {
 				"b": float64(-1),
 			},
 		},
+		{
+			Name:  "Key collision",
+			Input: `{"log.level":"info","log":{"source":"connectors-py-default"},"log":{"logger":"agent_component.cli"}}`,
+			Output: map[string]interface{}{
+				"log.level": "info",
+				"log": map[string]interface{}{
+					"logger": "agent_component.cli",
+				},
+			},
+		},
 	}
 
 	for _, test := range tests {
@@ -351,6 +361,51 @@ func TestMergeJSONFields(t *testing.T) {
 			JSONConfig:    Config{ExpandKeys: true, KeysUnderRoot: true},
 			ExpectedItems: mapstr.M{"a": mapstr.M{"b": mapstr.M{"c": "c", "d": "d"}}},
 		},
+		"key collision with expanded keys": {
+			Data: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+			JSONConfig: Config{ExpandKeys: true},
+			ExpectedItems: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+		},
+		"key collision without expanded keys": {
+			Data: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+			JSONConfig: Config{ExpandKeys: false},
+			ExpectedItems: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+		},
+		"key collision with overwrite": {
+			Data: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+			JSONConfig: Config{OverwriteKeys: true, AddErrorKey: true, IgnoreDecodingError: true},
+			ExpectedItems: mapstr.M{
+				"log.level": "info",
+				"log": mapstr.M{
+					"logger": "agent_component.cli",
+				},
+			},
+		},
 	}
 
 	for name, test := range tests {