Expose the With method of the zap logger (#9)

global.go

@@ -59,6 +59,12 @@ func Fatal(msg string, fields ...zap.Field) {
 	L().WithOptions(zap.AddCallerSkip(1)).Fatal(msg, fields...)
 }
 
+// With creates a child logger and adds structured context to it.
+// Fields added to the child don't affect the parent, and vice versa.
+func With(fields ...zap.Field) *zap.Logger {
+	return L().WithOptions(zap.AddCallerSkip(1)).With(fields...)
+}
+
 // SetLevel alters the logging level.
 func SetLevel(l zapcore.Level) {
 	_globalP.Level.SetLevel(l)

log_test.go

@@ -15,6 +15,7 @@ package log
 
 import (
 	. "github.com/pingcap/check"
+	"go.uber.org/zap"
 )
 
 var _ = Suite(&testLogSuite{})
@@ -30,4 +31,12 @@ func (t *testLogSuite) TestExport(c *C) {
 	Warn("Testing")
 	Error("Testing")
 	lg.AssertContains("log_test.go:")
+
+	lg = newZapTestLogger(conf, c)
+	ReplaceGlobals(lg.Logger, nil)
+	logger := With(zap.String("name", "tester"), zap.Int64("age", 42))
+	logger.Info("hello")
+	logger.Debug("world")
+	lg.AssertContains(`name=tester`)
+	lg.AssertContains(`age=42`)
 }