feat(log): sub-logger with prefix

Creates a sub-logger with the given prefix.

Fixes: #36

logger.go

@@ -264,6 +264,13 @@ func (l *Logger) With(keyvals ...interface{}) *Logger {
 	return &sl
 }
 
+// WithPrefix returns a new logger with the given prefix.
+func (l *Logger) WithPrefix(prefix string) *Logger {
+	sl := l.With()
+	sl.SetPrefix(prefix)
+	return sl
+}
+
 // Debug prints a debug message.
 func (l *Logger) Debug(msg interface{}, keyvals ...interface{}) {
 	l.log(DebugLevel, msg, keyvals...)

logger_test.go

@@ -125,3 +125,29 @@ func TestLogFormatter(t *testing.T) {
 		})
 	}
 }
+
+func TestLogWithPrefix(t *testing.T) {
+	var buf bytes.Buffer
+	cases := []struct {
+		name     string
+		expected string
+		prefix   string
+		msg      string
+	}{
+		{
+			name:     "with prefix",
+			expected: "INFO prefix: info\n",
+			prefix:   "prefix",
+			msg:      "info",
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			buf.Reset()
+			l := New(&buf)
+			l.SetPrefix(c.prefix)
+			l.Info(c.msg)
+			assert.Equal(t, c.expected, buf.String())
+		})
+	}
+}

pkg.go

@@ -112,6 +112,11 @@ func With(keyvals ...interface{}) *Logger {
 	return defaultLogger.With(keyvals...)
 }
 
+// WithPrefix returns a new logger with the given prefix.
+func WithPrefix(prefix string) *Logger {
+	return defaultLogger.WithPrefix(prefix)
+}
+
 // Helper marks the calling function as a helper
 // and skips it for source location information.
 // It's the equivalent of testing.TB.Helper().

pkg_test.go

@@ -203,3 +203,8 @@ func TestFormatter(t *testing.T) {
 	Info("info")
 	assert.Equal(t, "{\"lvl\":\"info\",\"msg\":\"info\"}\n", buf.String())
 }
+
+func TestWithPrefix(t *testing.T) {
+	l := WithPrefix("test")
+	assert.Equal(t, "test", l.prefix)
+}