structured logging: Add a list of fields to Metadata

It would be handy to have a list of fields, and some basic metadata about them (name, formatting options, etc) available in Metadata when structured logging is being used.

My desired use case here is something that is currently possible with tracing: In OpenVMM we have implemented a 'tagging' system for events, where by including a field with a known name and a value of tracing::Empty we can filter out the logging of events based on the presence or absence of these tags. See https://github.com/microsoft/openvmm/blob/main/vm/cvm_tracing/src/lib.rs if you're curious as to how this all works. And since tracing::Empty doesn't generate any output these tags are completely transparent to the final produced logs. I'm hoping Value::null() could be used for the same in log?

Start with Metadata and the structured-logging path, then compare the tagging approach in OpenVMM's vm/cvm_tracing/src/lib.rs. Define what field names and formatting metadata must be exposed, and verify whether Value::null() can represent transparent tag fields without changing produced logs.