zapcore: Add ParseLevel (#1047)

This adds the public ParseLevel function which enables the user to
construct a log level based on ASCII text input.

This re-uses the UnmarshalText method of Level for the heavy-lifting,
but it avoids the user having to declare a local `Level` variable first.


```go
var level zapcore.Level
err := level.UnmarshalText([]byte("debug"))

// versus
level, err := zapcore.ParseLevel("debug")
```

#### Usage

```go
level, err := zapcore.LevelFromString("info")
// zapcore.InfoLevel, nil

level, err := zapcore.LevelFromString("DEBUG")
// zapcore.DebugLevel, nil

level, err := zapcore.LevelFromString("FOO")
// 0, "invalid level"
```

Co-authored-by: Abhinav Gupta <abg@uber.com>

zapcore/level.go

@@ -55,6 +55,18 @@ const (
 	_maxLevel = FatalLevel
 )
 
+// ParseLevel parses a level based on the lower-case or all-caps ASCII
+// representation of the log level. If the provided ASCII representation is
+// invalid an error is returned.
+//
+// This is particularly useful when dealing with text input to configure log
+// levels.
+func ParseLevel(text string) (Level, error) {
+	var level Level
+	err := level.UnmarshalText([]byte(text))
+	return level, err
+}
+
 // String returns a lower-case ASCII representation of the log level.
 func (l Level) String() string {
 	switch l {

zapcore/level_test.go

@@ -76,6 +76,28 @@ func TestLevelText(t *testing.T) {
 	}
 }
 
+func TestParseLevel(t *testing.T) {
+	tests := []struct {
+		text  string
+		level Level
+		err   string
+	}{
+		{"info", InfoLevel, ""},
+		{"DEBUG", DebugLevel, ""},
+		{"FOO", 0, `unrecognized level: "FOO"`},
+	}
+	for _, tt := range tests {
+		parsedLevel, err := ParseLevel(tt.text)
+		if len(tt.err) == 0 {
+			assert.NoError(t, err)
+			assert.Equal(t, tt.level, parsedLevel)
+		} else {
+			assert.Error(t, err)
+			assert.Contains(t, err.Error(), tt.err)
+		}
+	}
+}
+
 func TestCapitalLevelsParse(t *testing.T) {
 	tests := []struct {
 		text  string