Add Func log method (rs#321)

This adds the Func log method to log using an anonymous function
only if the level is currently enabled.

The use case is for when you don't own an object and therefore can't
create your own marshaller but need to do some translation prior to
logging.

For example, this:

msg := log.Debug()
if msg.Enabled() {
  msg.Str("complicated_thing", makeBinaryThingLoggable(thing))
}
msg.Msg("Sending complicated thing")

Turns into this:

log.Debug().
  Func(func(e *Event) { e.Str("complicated_thing", makeBinaryThingLoggable(thing)) }).
  Msg("Sending complicated thing")

README.md

@@ -591,6 +591,7 @@ Some settings can be changed and will by applied to all loggers:
 ### Advanced Fields
 
 * `Err`: Takes an `error` and renders it as a string using the `zerolog.ErrorFieldName` field name.
+* `Func`: Run a `func` only if the level is enabled.
 * `Timestamp`: Inserts a timestamp field with `zerolog.TimestampFieldName` field name, formatted using `zerolog.TimeFieldFormat`.
 * `Time`: Adds a field with time formatted with `zerolog.TimeFieldFormat`.
 * `Dur`: Adds a field with `time.Duration`.

event.go

@@ -211,6 +211,14 @@ func (e *Event) Object(key string, obj LogObjectMarshaler) *Event {
 	return e
 }
 
+// Func allows an anonymous func to run only if the event is enabled.
+func (e *Event) Func(f func(e *Event)) *Event {
+	if e != nil && e.Enabled() {
+		f(e)
+	}
+	return e
+}
+
 // EmbedObject marshals an object that implement the LogObjectMarshaler interface.
 func (e *Event) EmbedObject(obj LogObjectMarshaler) *Event {
 	if e == nil {

log_test.go

@@ -260,6 +260,7 @@ func TestFields(t *testing.T) {
 		Bytes("bytes", []byte("bar")).
 		Hex("hex", []byte{0x12, 0xef}).
 		RawJSON("json", []byte(`{"some":"json"}`)).
+		Func(func(e *Event) { e.Str("func", "func_output") }).
 		AnErr("some_err", nil).
 		Err(errors.New("some error")).
 		Bool("bool", true).
@@ -283,7 +284,7 @@ func TestFields(t *testing.T) {
 		Time("time", time.Time{}).
 		TimeDiff("diff", now, now.Add(-10*time.Second)).
 		Msg("")
-	if got, want := decodeIfBinaryToString(out.Bytes()), `{"caller":"`+caller+`","string":"foo","stringer":"127.0.0.1","stringer_nil":null,"bytes":"bar","hex":"12ef","json":{"some":"json"},"error":"some error","bool":true,"int":1,"int8":2,"int16":3,"int32":4,"int64":5,"uint":6,"uint8":7,"uint16":8,"uint32":9,"uint64":10,"IPv4":"192.168.0.100","IPv6":"2001:db8:85a3::8a2e:370:7334","Mac":"00:14:22:01:23:45","Prefix":"192.168.0.100/24","float32":11.1234,"float64":12.321321321,"dur":1000,"time":"0001-01-01T00:00:00Z","diff":10000}`+"\n"; got != want {
+	if got, want := decodeIfBinaryToString(out.Bytes()), `{"caller":"`+caller+`","string":"foo","stringer":"127.0.0.1","stringer_nil":null,"bytes":"bar","hex":"12ef","json":{"some":"json"},"func":"func_output","error":"some error","bool":true,"int":1,"int8":2,"int16":3,"int32":4,"int64":5,"uint":6,"uint8":7,"uint16":8,"uint32":9,"uint64":10,"IPv4":"192.168.0.100","IPv6":"2001:db8:85a3::8a2e:370:7334","Mac":"00:14:22:01:23:45","Prefix":"192.168.0.100/24","float32":11.1234,"float64":12.321321321,"dur":1000,"time":"0001-01-01T00:00:00Z","diff":10000}`+"\n"; got != want {
 		t.Errorf("invalid log output:\ngot:  %v\nwant: %v", got, want)
 	}
 }
@@ -379,6 +380,7 @@ func TestFieldsDisabled(t *testing.T) {
 		Hex("hex", []byte{0x12, 0xef}).
 		AnErr("some_err", nil).
 		Err(errors.New("some error")).
+		Func(func(e *Event) { e.Str("func", "func_output") }).
 		Bool("bool", true).
 		Int("int", 1).
 		Int8("int8", 2).