Create metadata keys and contexts with LoggingContextFactory

[yevhenii-nadtochii]

This issue is a suggestion.

Now, metadata keys are created with LoggingFactory that also creates loggers.

To create a new context, one should use ScopedLoggingContext.newContext().

However, we have LoggingContextFactory that creates level maps and contexts. But it is internal.

Metadata keys strictly relate to the context. So, I suggest moving keys and context creation to LoggingContextFactory, and make this factory public.