Allow constomizing the log format produced by `Logging{Service,Client}`

The logs produced by LoggingService and LoggingClient are human-readable. However, this non-structured text is not good when users want to store the log file to Elastic Search or other key-value storage. We can give an extension API to users to customize the output format. For example:

interface LogFormat {
    static LogFormat ofText() {
        // return a log message formatted by the default formatter. 
    }

    static LogFormat ofJson() {
        // return a log message serialized to JSON format
    }

    String format(RequestLog log, LogSanitizers sanitizers);
}

class LogSanitizers {
    BiFunction<? super RequestContext, ? super RequestHeaders, ? extends @Nullable Object> requestHeadersSanitizer;
    BiFunction<? super RequestContext, Object, ? extends @Nullable Object> requestContentSanitizer;
    ...
}

LoggingService.builder()
              // Customize the log message format using LogFormat
              .format(LogFormat.ofJson())

[ikhoon]

hasInterestedFlags(flags, RequestLogProperty.REQUEST_LENGTH) must be the same with RequestLog.isAvailable(RequestLogProperty.REQUEST_LENGTH). By the way, flags is a volatile value. So it is bad to access it repeatedly. How about adding an API that returns all available RequestLogPropertys?

interface RequestLogAccess {
    Set<RequestLogProperty> availableProperties();
}

[ikhoon]

Sure ❤️