feat: add logger interface and default implementation

Author: sagikazarmark

logger.go

@@ -0,0 +1,77 @@
+package viper
+
+import (
+	"fmt"
+
+	jww "github.com/spf13/jwalterweatherman"
+)
+
+// Logger is a unified interface for various logging use cases and practices, including:
+// 		- leveled logging
+// 		- structured logging
+type Logger interface {
+	// Trace logs a Trace event.
+	//
+	// Even more fine-grained information than Debug events.
+	// Loggers not supporting this level should fall back to Debug.
+	Trace(msg string, keyvals ...interface{})
+
+	// Debug logs a Debug event.
+	//
+	// A verbose series of information events.
+	// They are useful when debugging the system.
+	Debug(msg string, keyvals ...interface{})
+
+	// Info logs an Info event.
+	//
+	// General information about what's happening inside the system.
+	Info(msg string, keyvals ...interface{})
+
+	// Warn logs a Warn(ing) event.
+	//
+	// Non-critical events that should be looked at.
+	Warn(msg string, keyvals ...interface{})
+
+	// Error logs an Error event.
+	//
+	// Critical events that require immediate attention.
+	// Loggers commonly provide Fatal and Panic levels above Error level,
+	// but exiting and panicing is out of scope for a logging library.
+	Error(msg string, keyvals ...interface{})
+}
+
+type jwwLogger struct{}
+
+func (jwwLogger) Trace(msg string, keyvals ...interface{}) {
+	jww.TRACE.Printf(jwwLogMessage(msg, keyvals...))
+}
+
+func (jwwLogger) Debug(msg string, keyvals ...interface{}) {
+	jww.DEBUG.Printf(jwwLogMessage(msg, keyvals...))
+}
+
+func (jwwLogger) Info(msg string, keyvals ...interface{}) {
+	jww.INFO.Printf(jwwLogMessage(msg, keyvals...))
+}
+
+func (jwwLogger) Warn(msg string, keyvals ...interface{}) {
+	jww.WARN.Printf(jwwLogMessage(msg, keyvals...))
+}
+
+func (jwwLogger) Error(msg string, keyvals ...interface{}) {
+	jww.ERROR.Printf(jwwLogMessage(msg, keyvals...))
+}
+
+func jwwLogMessage(msg string, keyvals ...interface{}) string {
+	out := msg
+
+	if len(keyvals) > 0 && len(keyvals)%2 == 1 {
+		keyvals = append(keyvals, nil)
+	}
+
+	for i := 0; i <= len(keyvals)-2; i += 2 {
+		out = fmt.Sprintf("%s %v=%v", out, keyvals[i], keyvals[i+1])
+	}
+
+	return out
+}