re-wire logging on top of zerolog

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

cmd/dagger/logger/logger.go

@@ -0,0 +1,51 @@
+// Logger utilities for the CLI
+//
+// These utilities rely on command line flags to set up the logger, therefore
+// they are tightly coupled with the CLI and should not be used outside.
+
+package logger
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/rs/zerolog"
+	"github.com/spf13/viper"
+	"golang.org/x/term"
+)
+
+func New() zerolog.Logger {
+	logger := zerolog.
+		New(os.Stderr).
+		With().
+		Timestamp().
+		Logger()
+
+	if prettyLogs() {
+		logger = logger.Output(zerolog.ConsoleWriter{Out: os.Stderr})
+	} else {
+		logger = logger.With().Timestamp().Caller().Logger()
+	}
+
+	level := viper.GetString("log-level")
+	lvl, err := zerolog.ParseLevel(level)
+	if err != nil {
+		panic(err)
+	}
+	return logger.Level(lvl)
+}
+
+func prettyLogs() bool {
+	switch f := viper.GetString("log-format"); f {
+	case "json":
+		return false
+	case "pretty":
+		return true
+	case "":
+		return term.IsTerminal(int(os.Stdout.Fd()))
+	default:
+		fmt.Fprintf(os.Stderr, "invalid --log-format %q\n", f)
+		os.Exit(1)
+	}
+	return false
+}