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

cmd/mattermost/commands/logs.go

@@ -0,0 +1,52 @@
+// Copyright (c) 2016-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package commands
+
+import (
+	"github.com/mattermost/mattermost-server/mlog/human"
+	"github.com/spf13/cobra"
+	"io"
+	"os"
+)
+
+var LogsCmd = &cobra.Command{
+	Use:   "logs",
+	Short: "Display logs in a human-readable format",
+	RunE:  logsCmdF,
+}
+
+func init() {
+	LogsCmd.Flags().Bool("logrus", false, "Use logrus for formatting.")
+	RootCmd.AddCommand(LogsCmd)
+}
+
+func logsCmdF(command *cobra.Command, args []string) error {
+	// check stdin to see if we have a pipe
+	fi, err := os.Stdin.Stat()
+	if err != nil {
+		return err
+	}
+
+	var input io.Reader
+	if fi.Size() == 0 && fi.Mode()&os.ModeNamedPipe == 0 {
+		file, err := os.Open("mattermost.log")
+		if err != nil {
+			return err
+		}
+		defer file.Close()
+		input = file
+	} else {
+		input = os.Stdin
+	}
+	var writer human.LogWriter
+
+	if flag, _ := command.Flags().GetBool("logrus"); flag {
+		writer = human.NewLogrusWriter(os.Stdout)
+	} else {
+		writer = human.NewSimpleWriter(os.Stdout)
+	}
+	human.ProcessLogs(input, writer)
+
+	return nil
+}