example: log-extension

[virkt25]

Use the https://github.com/strongloop/loopback4-extension-starter repo to create an example logging extension that has the following:

• Mixin to set level that should be logged.
• Decorator to mark the log level for individual methods
• Action for logging, default logging level, timer, etc.
• Document the process.
• Tests

Repo: https://github.com/strongloop/loopback4-example-log-extension

This is a follow up task from #525 -> in particular discussion under strongloop-archive/loopback4-extension-starter#1