W3CLoggerProcessor doesn't update fields directive when it changes

FileLoggerProcessor rolls to a new file & updates the fields set when it changes:

https://github.com/dotnet/aspnetcore/blob/b083d7de3981b1ac3dc2fdc72a53e5ad07172567/src/Middleware/HttpLogging/src/FileLoggerProcessor.cs#L64-L72

But W3CLoggerProcessor doesn't. We should fix this for 7 - maybe just pass in the set of fields directly from the middleware every time. Example proposed change set from @halter73: https://github.com/dotnet/aspnetcore/compare/dotnet:b806844...dotnet:a4d9f19?expand=1