Masks which only allow messages matching the corresponding severity level to be processed. These values would be passed to SetLogMask.
GetFromEnv gives the Mask indicated by environment variables (or else the default Mask).
UpTo creates a Mask that unmasks all log priorities up the given pri.Priority.
Masked indicates whether a log message with the given pri.Priority should be masked (i.e. hidden) by this Mask.
String creates a string representation of the Mask.