feat(logging): Allow overriding message key for structured logging (#16242)

cmd/telegraf/agent.conf

@@ -57,6 +57,10 @@
   ## "structured" or, on Windows, "eventlog".
   # logformat = "text"
 
+  ## Message key for structured logs, to override the default of "msg".
+  ## Ignored if `logformat` is not "structured".
+  # structured_log_message_key = "message"
+
   ## Name of the file to be logged to or stderr if unset or empty. This
   ## setting is ignored for the "eventlog" format.
   # logfile = ""

cmd/telegraf/telegraf.go

@@ -369,6 +369,7 @@ func (t *Telegraf) runAgent(ctx context.Context, reloadConfig bool) error {
 		LogTarget:               c.Agent.LogTarget,
 		LogFormat:               c.Agent.LogFormat,
 		Logfile:                 c.Agent.Logfile,
+		StructuredLogMessageKey: c.Agent.StructuredLogMessageKey,
 		RotationInterval:        time.Duration(c.Agent.LogfileRotationInterval),
 		RotationMaxSize:         int64(c.Agent.LogfileRotationMaxSize),
 		RotationMaxArchives:     c.Agent.LogfileRotationMaxArchives,

config/config.go

@@ -236,6 +236,10 @@ type AgentConfig struct {
 	// Name of the file to be logged to or stderr if empty. Ignored for "eventlog" format.
 	Logfile string `toml:"logfile"`
 
+	// Message key for structured logs, to override the default of "msg".
+	// Ignored if "logformat" is not "structured".
+	StructuredLogMessageKey string `toml:"structured_log_message_key"`
+
 	// The file will be rotated after the time interval specified.  When set
 	// to 0 no time based rotation is performed.
 	LogfileRotationInterval Duration `toml:"logfile_rotation_interval"`

docs/CONFIGURATION.md

@@ -307,6 +307,10 @@ The agent table configures Telegraf and the defaults used across all plugins.
   "structured" or, on Windows, "eventlog". The output file (if any) is
   determined by the `logfile` setting.
 
+- **structured_log_message_key**:
+  Message key for structured logs, to override the default of "msg".
+  Ignored if `logformat` is not "structured".
+
 - **logfile**:
   Name of the file to be logged to or stderr if unset or empty. This
   setting is ignored for the "eventlog" format.

logger/logger.go

@@ -195,6 +195,8 @@ type Config struct {
 	LogWithTimezone string
 	// Logger instance name
 	InstanceName string
+	// Structured logging message key
+	StructuredLogMessageKey string
 
 	// internal  log-level
 	logLevel telegraf.LogLevel

logger/structured_logger.go

@@ -41,9 +41,7 @@ func (l *structuredLogger) Print(level telegraf.LogLevel, ts time.Time, _ string
 	}
 }
 
-var defaultStructuredHandlerOptions = &slog.HandlerOptions{
+var defaultReplaceAttr = func(_ []string, attr slog.Attr) slog.Attr {
-	Level: slog.Level(-99),
-	ReplaceAttr: func(_ []string, attr slog.Attr) slog.Attr {
 	// Translate the Telegraf log-levels to strings
 	if attr.Key == slog.LevelKey {
 		if level, ok := attr.Value.Any().(slog.Level); ok {
@@ -51,7 +49,11 @@ var defaultStructuredHandlerOptions = &slog.HandlerOptions{
 		}
 	}
 	return attr
-	},
+}
+
+var defaultStructuredHandlerOptions = &slog.HandlerOptions{
+	Level:       slog.Level(-99),
+	ReplaceAttr: defaultReplaceAttr,
 }
 
 func init() {
@@ -70,8 +72,20 @@ func init() {
 			writer = w
 		}
 
+		structuredHandlerOptions := defaultStructuredHandlerOptions
+
+		if cfg.StructuredLogMessageKey != "" {
+			structuredHandlerOptions.ReplaceAttr = func(groups []string, attr slog.Attr) slog.Attr {
+				if attr.Key == slog.MessageKey {
+					attr.Key = cfg.StructuredLogMessageKey
+				}
+
+				return defaultReplaceAttr(groups, attr)
+			}
+		}
+
 		return &structuredLogger{
-			handler: slog.NewJSONHandler(writer, defaultStructuredHandlerOptions),
+			handler: slog.NewJSONHandler(writer, structuredHandlerOptions),
 			output:  writer,
 			errlog:  log.New(os.Stderr, "", 0),
 		}, nil

logger/structured_logger_test.go

@@ -307,6 +307,44 @@ func TestStructuredWriteToFileInRotation(t *testing.T) {
 	require.Len(t, files, 2)
 }
 
+func TestStructuredLogMessageKey(t *testing.T) {
+	instance = defaultHandler()
+
+	tmpfile, err := os.CreateTemp("", "")
+	require.NoError(t, err)
+	defer os.Remove(tmpfile.Name())
+
+	cfg := &Config{
+		Logfile:                 tmpfile.Name(),
+		LogFormat:               "structured",
+		RotationMaxArchives:     -1,
+		Debug:                   true,
+		StructuredLogMessageKey: "message",
+	}
+	require.NoError(t, SetupLogging(cfg))
+
+	l := New("testing", "test", "")
+	l.Info("TEST")
+
+	buf, err := os.ReadFile(tmpfile.Name())
+	require.NoError(t, err)
+
+	expected := map[string]interface{}{
+		"level":    "INFO",
+		"message":  "TEST",
+		"category": "testing",
+		"plugin":   "test",
+	}
+
+	var actual map[string]interface{}
+	require.NoError(t, json.Unmarshal(buf, &actual))
+
+	require.Contains(t, actual, "time")
+	require.NotEmpty(t, actual["time"])
+	delete(actual, "time")
+	require.Equal(t, expected, actual)
+}
+
 func BenchmarkTelegrafStructuredLogWrite(b *testing.B) {
 	// Discard all logging output
 	l := &structuredLogger{