[doc][log] Add survey for apex/log #26
[doc][log] Add survey for apex/log #26

- it use lock in handler implementations, for logging to stdout/err, I
think the reason the lock is needed is they call fmt.Fprintf
multiple times (for the message and fields) for a single log entry
- the overall process is entry.Debug -> Logger.log ->
Handler.HandleLog(*Entry) -> write to ouput (i.e.
fmt.Fprintf(io.Writer))
- it's good they didn't put Output inside logger, als othe HandlerFunc
part like `net/http`, though I still didn't know why it's using value
instead of pointer receiver #30

dyweb/gommon,dyweb/gommon,dyweb/gommon