feat(miniooni): optionally log using emojis (#932)

As silly as it seems, emojis help _a lot_ when eyeballing logs
to quickly identify unexpected lines.

I'm doing this work as part of https://github.com/ooni/probe/issues/2257

internal/logx/logx.go

@@ -0,0 +1,63 @@
+// Package logx contains logging extensions
+package logx
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"time"
+
+	"github.com/apex/log"
+)
+
+// Handler implements github.com/apex/log.Handler.
+type Handler struct {
+	// Emoji is OPTIONAL and indicates whether to enable emojis.
+	Emoji bool
+
+	// Now is the MANDATORY function to compute the current time.
+	Now func() time.Time
+
+	// StartTime is MANDATORY and indicates when we started logging.
+	StartTime time.Time
+
+	// Writer is MANDATORY and is the underlying writer.
+	io.Writer
+}
+
+// NewHandlerWithDefaultSettings creates a new Handler with default settings.
+func NewHandlerWithDefaultSettings() *Handler {
+	return &Handler{
+		Emoji:     false,
+		Now:       time.Now,
+		StartTime: time.Now(),
+		Writer:    os.Stderr,
+	}
+}
+
+var _ log.Handler = &Handler{}
+
+// HandleLog implements log.Handler
+func (h *Handler) HandleLog(e *log.Entry) (err error) {
+	level := fmt.Sprintf("<%s>", e.Level.String())
+	if h.Emoji {
+		switch e.Level {
+		case log.DebugLevel:
+			level = "🧐"
+		case log.InfoLevel:
+			level = "🗒️"
+		case log.WarnLevel:
+			level = "🔥"
+		default:
+			// keep the original string
+		}
+	}
+	elapsed := h.Now().Sub(h.StartTime)
+	s := fmt.Sprintf("[%14.6f] %s %s", elapsed.Seconds(), level, e.Message)
+	if len(e.Fields) > 0 {
+		s += fmt.Sprintf(": %+v", e.Fields)
+	}
+	s += "\n"
+	_, err = h.Writer.Write([]byte(s))
+	return
+}