feat(logger): add runtime log level control API

Add independent log level control for stderr and OTLP loggers.
Both can be configured via environment variables or programmatically
at runtime.

- Add SetLevel() and SetOTLPLevel() for runtime control
- Add ParseLevel() to convert strings to slog.Level
- Support LOG_LEVEL and OTLP_LOG_LEVEL env vars
- Maintain backward compatibility with DEBUG env var
- Add comprehensive test coverage

logger/logger.go

@@ -18,12 +18,15 @@
 //   - Context propagation for request-scoped logging
 //
 // Environment variables:
-//   - DEBUG: Enable debug level logging (configurable prefix via ConfigPrefix)
+//   - LOG_LEVEL: Set stderr log level (DEBUG, INFO, WARN, ERROR) (configurable prefix via ConfigPrefix)
+//   - OTLP_LOG_LEVEL: Set OTLP log level independently (configurable prefix via ConfigPrefix)
+//   - DEBUG: Enable debug level logging for backward compatibility (configurable prefix via ConfigPrefix)
 //   - INVOCATION_ID: Systemd detection for timestamp handling
 package logger
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"log/slog"
 	"os"
@@ -43,6 +46,16 @@ import (
 // This enables multiple services to have independent logging configuration.
 var ConfigPrefix = ""
 
+var (
+	// Level controls the log level for the default stderr logger.
+	// Can be changed at runtime to adjust logging verbosity.
+	Level = new(slog.LevelVar) // Info by default
+
+	// OTLPLevel controls the log level for OTLP output.
+	// Can be changed independently from the stderr logger level.
+	OTLPLevel = new(slog.LevelVar) // Info by default
+)
+
 var (
 	textLogger  *slog.Logger
 	otlpLogger  *slog.Logger
@@ -56,21 +69,64 @@ var (
 	mu         sync.Mutex
 )
 
-func setupStdErrHandler() slog.Handler {
-	programLevel := new(slog.LevelVar) // Info by default
+// SetLevel sets the log level for the default stderr logger.
+// This affects the primary application logger returned by Setup().
+func SetLevel(level slog.Level) {
+	Level.Set(level)
+}
 
-	envVar := "DEBUG"
+// SetOTLPLevel sets the log level for OTLP output.
+// This affects the logger returned by SetupOLTP() and the OTLP portion of SetupMultiLogger().
+func SetOTLPLevel(level slog.Level) {
+	OTLPLevel.Set(level)
+}
+
+// ParseLevel converts a string log level to slog.Level.
+// Supported levels: "DEBUG", "INFO", "WARN", "ERROR" (case insensitive).
+// Returns an error for unrecognized level strings.
+func ParseLevel(level string) (slog.Level, error) {
+	switch {
+	case level == "":
+		return slog.LevelInfo, nil
+	case level == "DEBUG" || level == "debug":
+		return slog.LevelDebug, nil
+	case level == "INFO" || level == "info":
+		return slog.LevelInfo, nil
+	case level == "WARN" || level == "warn":
+		return slog.LevelWarn, nil
+	case level == "ERROR" || level == "error":
+		return slog.LevelError, nil
+	default:
+		return slog.LevelInfo, fmt.Errorf("unknown log level: %s", level)
+	}
+}
+
+func setupStdErrHandler() slog.Handler {
+	// Parse LOG_LEVEL environment variable
+	logLevelVar := "LOG_LEVEL"
 	if len(ConfigPrefix) > 0 {
-		envVar = ConfigPrefix + "_" + envVar
+		logLevelVar = ConfigPrefix + "_" + logLevelVar
 	}
 
-	if opt := os.Getenv(envVar); len(opt) > 0 {
-		if debug, _ := strconv.ParseBool(opt); debug {
-			programLevel.Set(slog.LevelDebug)
+	if levelStr := os.Getenv(logLevelVar); levelStr != "" {
+		if level, err := ParseLevel(levelStr); err == nil {
+			Level.Set(level)
 		}
 	}
 
-	logOptions := &slog.HandlerOptions{Level: programLevel}
+	// Maintain backward compatibility with DEBUG environment variable
+	debugVar := "DEBUG"
+	if len(ConfigPrefix) > 0 {
+		debugVar = ConfigPrefix + "_" + debugVar
+	}
+
+	if opt := os.Getenv(debugVar); len(opt) > 0 {
+		if debug, _ := strconv.ParseBool(opt); debug {
+			Level.Set(slog.LevelDebug)
+		}
+	}
+
+	logOptions := &slog.HandlerOptions{Level: Level}
 
 	if len(os.Getenv("INVOCATION_ID")) > 0 {
 		// don't add timestamps when running under systemd
@@ -88,6 +144,18 @@ func setupStdErrHandler() slog.Handler {
 
 func setupOtlpLogger() *slog.Logger {
 	setupOtlp.Do(func() {
+		// Parse OTLP_LOG_LEVEL environment variable
+		otlpLevelVar := "OTLP_LOG_LEVEL"
+		if len(ConfigPrefix) > 0 {
+			otlpLevelVar = ConfigPrefix + "_" + otlpLevelVar
+		}
+
+		if levelStr := os.Getenv(otlpLevelVar); levelStr != "" {
+			if level, err := ParseLevel(levelStr); err == nil {
+				OTLPLevel.Set(level)
+			}
+		}
+
 		// Create our buffering exporter
 		// It will buffer until tracing is configured
 		bufferingExp := newBufferingExporter()
@@ -107,8 +175,9 @@ func setupOtlpLogger() *slog.Logger {
 		// Set global provider
 		global.SetLoggerProvider(provider)
 
-		// Create slog handler
-		handler := newLogFmtHandler(otelslog.NewHandler("common"))
+		// Create slog handler with level control
+		baseHandler := newLogFmtHandler(otelslog.NewHandler("common"))
+		handler := newOTLPLevelHandler(baseHandler)
 		otlpLogger = slog.New(handler)
 	})
 	return otlpLogger