Filter fields from parent

[Sytten]

Hi! Currently all fields from the parent span are added to the child span. This means you can get a lot of not so useful fields when running this with the actix middleware, personally I would only like to have the request id (no need to repeat the ip, path, user agent, etc.). So I think a filtering system could be good, I am just unsure how to make it flexible enough. Like different filters for different parent span. Though we could also start with just a basic filter. Happy to help on the implementation Thanks :)

[Sytten]

Just checking again to see if someone could look into my PR. Thanks!