structured logging: auto-populate log messages with additional metadata

[minherz]

Is your feature request related to a problem? Please describe.
Logs written by LoggingAppender require developing an enhancer to auto populating missing metadata (per entry) such as source location, http request and tracing information, etc. In result the user experience when using Log Explorer and other Ops tools is poor.

Describe the solution you'd like
Use java-logging API such as populateMetadata() to support this functionality in the logback appender.
Add an opt-out configuration to disable automatic population per appender.

[minherz]

Related issues #552, #553, #554, #555, googleapis/java-logging#685, googleapis/java-logging#686, googleapis/java-logging#687, googleapis/java-logging#688, googleapis/java-logging#689, googleapis/java-logging#690, googleapis/java-logging#691

[minherz]

Somehow this issue was not automatically fixed although it was referenced in the multiple PRs. Close it now.