class type logger =object..end
A logger receives log messages
method log_subch : component:string ->
       subchannel:string -> level:level -> message:string -> unitReceive a log message of level from component. The component
        string is the name of the socket service emitting the message.
        Optionally, one can specify a subchannel when a single component
        needs to write to several log files. For example, a subchannel
        of "access" could select the access log of a webserver.
        The main log file is selected by passing the empty string as
        subchannel.
method log : component:string -> level:level -> message:string -> unitSame as log_subch when subchannel is set to the empty string.
        This means that the message is sent to the main log file of the
        component.
method reopen : unit -> unitReopen the log files