Refactor logging in more Go idiomatic style (#6816)

This refactor brings a change which allows
targets to be added in a cleaner way and also
audit is now moved out.

This PR also simplifies logger dependency for auditing

cmd/logger/target/console/console.go

@@ -0,0 +1,109 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package console
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/minio/minio/cmd/logger"
+	"github.com/minio/minio/cmd/logger/message/log"
+)
+
+// Target implements loggerTarget to send log
+// in plain or json format to the standard output.
+type Target struct{}
+
+// Send log message 'e' to console
+func (c *Target) Send(e interface{}) error {
+	entry, ok := e.(log.Entry)
+	if !ok {
+		return fmt.Errorf("Uexpected log entry structure %#v", e)
+	}
+	if logger.IsJSON() {
+		logJSON, err := json.Marshal(&entry)
+		if err != nil {
+			return err
+		}
+		fmt.Println(string(logJSON))
+		return nil
+	}
+
+	trace := make([]string, len(entry.Trace.Source))
+
+	// Add a sequence number and formatting for each stack trace
+	// No formatting is required for the first entry
+	for i, element := range entry.Trace.Source {
+		trace[i] = fmt.Sprintf("%8v: %s", i+1, element)
+	}
+
+	tagString := ""
+	for key, value := range entry.Trace.Variables {
+		if value != "" {
+			if tagString != "" {
+				tagString += ", "
+			}
+			tagString += key + "=" + value
+		}
+	}
+
+	apiString := "API: " + entry.API.Name + "("
+	if entry.API.Args != nil && entry.API.Args.Bucket != "" {
+		apiString = apiString + "bucket=" + entry.API.Args.Bucket
+	}
+	if entry.API.Args != nil && entry.API.Args.Object != "" {
+		apiString = apiString + ", object=" + entry.API.Args.Object
+	}
+	apiString += ")"
+	timeString := "Time: " + time.Now().Format(logger.TimeFormat)
+
+	var requestID string
+	if entry.RequestID != "" {
+		requestID = "\nRequestID: " + entry.RequestID
+	}
+
+	var remoteHost string
+	if entry.RemoteHost != "" {
+		remoteHost = "\nRemoteHost: " + entry.RemoteHost
+	}
+
+	var userAgent string
+	if entry.UserAgent != "" {
+		userAgent = "\nUserAgent: " + entry.UserAgent
+	}
+
+	if len(entry.Trace.Variables) > 0 {
+		tagString = "\n       " + tagString
+	}
+
+	var msg = logger.ColorFgRed(logger.ColorBold(entry.Trace.Message))
+	var output = fmt.Sprintf("\n%s\n%s%s%s%s\nError: %s%s\n%s",
+		apiString, timeString, requestID, remoteHost, userAgent,
+		msg, tagString, strings.Join(trace, "\n"))
+
+	fmt.Println(output)
+	return nil
+}
+
+// New initializes a new logger target
+// which prints log directly in the standard
+// output.
+func New() *Target {
+	return &Target{}
+}