FromContext 从Context中提取Logger，如果不存在则返回默认的Logger。.

FromContextAsHelper 从Context中提取Logger，返回Helper对象。.

InContext 将Logger存入Context。.

NewStdLogger new a logger implements Logger interface.

DefaultCaller is a Valuer that returns the file and line where the Log method was invoked.

ErrMissingValue is appended to key-values slices with odd length to substitute the missing value.