Allow customizing written log entries by exposing parameters via appender configuration

[minherz]

Is your feature request related to a problem? Please describe.
Writing log entries can be customized by providing user metadata as labels (a map of key, value pairs), customizing the logger name and which project it is located. The data cannot be customized when logging using the appender.

Describe the solution you'd like
It would be great to use logback configuration to provide these values (user labels, log name and custom project location) per appender instance.

Describe alternatives you've considered
There are no good alternative if the logback appender is used.

Additional context
There is PR #371 that partially addresses the problem. Unfortunately, the current set of WriteOption options does not allow to customize the log location for the LOG_NAME option.

[minherz]

Follow up on googleapis/java-logging#684 before implementing this.