Implement structured logs using logrus

This was based loosely on an earlier implementation by
Danny Kopping <danny.kopping@grafana.com>

logger.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	log *logrus.Logger
+)
+
+func setupLogger() {
+	log = logrus.New()
+
+	// Handle logfile
+	if (*logFile != "") {
+		writer, err := os.OpenFile(*logFile, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0600)
+		if err != nil {
+			fmt.Printf("cannot open log file: %s\n", err)
+			os.Exit(1)
+		}
+
+		log.SetOutput(writer)
+	}
+
+	// Handle log_format
+	switch *logFormat {
+	case "json":
+		log.SetFormatter(&logrus.JSONFormatter{
+			TimestampFormat:   time.RFC3339Nano,
+			DisableHTMLEscape: true,
+		})
+	case "plain":
+		log.SetFormatter(&logrus.TextFormatter{
+			DisableTimestamp: true,
+		})
+	case "", "default":
+		log.SetFormatter(&logrus.TextFormatter{
+			FullTimestamp: true,
+		})
+	default:
+		fmt.Fprintf(os.Stderr, "Invalid log_format: %s\n", *logFormat)
+		os.Exit(1)
+	}
+
+	// Handle log_level
+	level, err := logrus.ParseLevel(*logLevel)
+	if err != nil {
+		level = logrus.InfoLevel
+
+		log.WithField("given_level", *logLevel).
+			Warn("could not parse log level, defaulting to 'info'")
+	}
+	log.SetLevel(level)
+}