Add library and command for human-readable logs (#9809)

* Update logrus to 1.2 and add as a direct dependency

* Create an mlog/human package for pretty-printing logs

This package can read JSON logs from mattermost.log, and output the data to
either logrus or a custom formatter, to make the logs more human readable.

* Create a command for outputting human-readable logs

This command will read JSON data from mattermost.log or stdin, and
output in a human readable format. An optional argument can be used
to activate logrus output (which includes color support).

* Reorganize code in mlog/human and improve logrus timestamp formatting

mlog/human/entry.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package human
+
+import (
+	"fmt"
+	"github.com/mattermost/mattermost-server/mlog"
+	"strings"
+	"time"
+)
+
+type LogEntry struct {
+	Time    time.Time
+	Level   string
+	Message string
+	Caller  string
+	Fields  []mlog.Field
+}
+
+// Provide default string representation. Used by SimpleWriter
+func (f LogEntry) String() string {
+	var sb strings.Builder
+	if !f.Time.IsZero() {
+		sb.WriteString(f.Time.Format(time.RFC3339Nano))
+		sb.WriteRune(' ')
+	}
+	if f.Level != "" {
+		sb.WriteString(f.Level)
+		sb.WriteRune(' ')
+	}
+	if f.Caller != "" {
+		sb.WriteString(f.Caller)
+		sb.WriteRune(' ')
+	}
+	for _, field := range f.Fields {
+		sb.WriteString(field.Key)
+		sb.WriteRune('=')
+		sb.WriteString(fmt.Sprint(field.Interface))
+		sb.WriteRune(' ')
+	}
+	if f.Message != "" {
+		// If the message is multiple lines, start the whole message on a new line
+		if strings.ContainsRune(f.Message, '\n') {
+			sb.WriteRune('\n')
+		}
+		sb.WriteString(f.Message)
+	}
+
+	return sb.String()
+}