zap: Add ParseAtomicLevel func (#1048)

As discussed in #1047 this PR adds a zap.ParseAtomicLevel() func which enables the user to construct a log level based on ASCII text input.

Usage
```
level, err := zap.ParseAtomicLevel("info") // InfoLevel, nil
level, err := zap.ParseAtomicLevel("DEBUG") // DebugLevel, nil
level, err := zap.ParseAtomicLevel("FOO") // InfoLevel, "unrecognized level: "FOO""
```

Co-authored-by: Sung Yoon Whang <sungyoonwhang@gmail.com>

Co-authored-by: Sung Yoon Whang <sungyoonwhang@gmail.com>

level.go

@@ -86,6 +86,23 @@ func NewAtomicLevelAt(l zapcore.Level) AtomicLevel {
 	return a
 }
 
+// ParseAtomicLevel parses an AtomicLevel based on a lowercase 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 ParseAtomicLevel(text string) (AtomicLevel, error) {
+	a := NewAtomicLevel()
+	l, err := zapcore.ParseLevel(text)
+	if err != nil {
+		return a, err
+	}
+
+	a.SetLevel(l)
+	return a, nil
+}
+
 // Enabled implements the zapcore.LevelEnabler interface, which allows the
 // AtomicLevel to be used in place of traditional static levels.
 func (lvl AtomicLevel) Enabled(l zapcore.Level) bool {

level_test.go

@@ -57,6 +57,29 @@ func TestNewAtomicLevel(t *testing.T) {
 	assert.Equal(t, WarnLevel, lvl.Level(), "Unexpected level after SetLevel.")
 }
 
+func TestParseAtomicLevel(t *testing.T) {
+	tests := []struct {
+		text  string
+		level AtomicLevel
+		err   string
+	}{
+		{"info", NewAtomicLevel(), ""},
+		{"DEBUG", NewAtomicLevelAt(DebugLevel), ""},
+		{"FOO", NewAtomicLevel(), `unrecognized level: "FOO"`},
+	}
+
+	for _, tt := range tests {
+		parsedAtomicLevel, err := ParseAtomicLevel(tt.text)
+		if len(tt.err) == 0 {
+			assert.NoError(t, err)
+			assert.Equal(t, tt.level, parsedAtomicLevel)
+		} else {
+			assert.Error(t, err)
+			assert.Contains(t, err.Error(), tt.err)
+		}
+	}
+}
+
 func TestAtomicLevelMutation(t *testing.T) {
 	lvl := NewAtomicLevel()
 	lvl.SetLevel(WarnLevel)