Fix logger after filter api change in luci-go.

impl/prod/logger.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package prod
 
 import (
         "fmt"
 
         "github.com/luci/luci-go/common/logging"
@@ skipping 43 matching lines @@
                 logf = log.Warningf
 
         case logging.Error:
                 fallthrough
         default:
                 logf = log.Errorf
         }
 
         fields := logging.GetFields(gl.ic)
         if len(fields) > 0 {
-                logf(gl.aeCtx, "%s :: %s", fmt.Sprintf(format, args...), fields.FieldString(true))
+                logf(gl.aeCtx, "%s :: %s", fmt.Sprintf(format, args...), fields.String())
         } else {
                 logf(gl.aeCtx, format, args...)
         }
 }