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/audit.go

@@ -0,0 +1,68 @@
+/*
+ * 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 logger
+
+import (
+	"net/http"
+
+	"github.com/minio/minio/cmd/logger/message/audit"
+)
+
+// ResponseWriter - is a wrapper to trap the http response status code.
+type ResponseWriter struct {
+	http.ResponseWriter
+	statusCode int
+}
+
+// NewResponseWriter - returns a wrapped response writer to trap
+// http status codes for auditiing purposes.
+func NewResponseWriter(w http.ResponseWriter) *ResponseWriter {
+	return &ResponseWriter{w, http.StatusOK}
+}
+
+// WriteHeader - writes http status code
+func (lrw *ResponseWriter) WriteHeader(code int) {
+	lrw.statusCode = code
+	lrw.ResponseWriter.WriteHeader(code)
+}
+
+// Flush - Calls the underlying Flush.
+func (lrw *ResponseWriter) Flush() {
+	lrw.ResponseWriter.(http.Flusher).Flush()
+}
+
+// AuditTargets is the list of enabled audit loggers
+var AuditTargets = []Target{}
+
+// AddAuditTarget adds a new audit logger target to the
+// list of enabled loggers
+func AddAuditTarget(t Target) {
+	AuditTargets = append(AuditTargets, t)
+}
+
+// AuditLog - logs audit logs to all audit targets.
+func AuditLog(w http.ResponseWriter, r *http.Request, api string) {
+	var statusCode int
+	lrw, ok := w.(*ResponseWriter)
+	if ok {
+		statusCode = lrw.statusCode
+	}
+	// Send audit logs only to http targets.
+	for _, t := range AuditTargets {
+		t.Send(audit.ToEntry(w, r, api, statusCode))
+	}
+}