Add support for including metadata in requests tapped by tap filter

[onemanbucket]

Title: Add support for including metadata in requests tapped by tap filter

Description:
We use the tap filter to sample perimeter traffic for building a traffic insights dataset. We want to attach some information to the sampled requests, so we set some route metadata which is then added as a request header. This lets us attach metadata to the request sampled by the tap filter, but it has the downside that the metadata headers are sent to our upstreams which leaks internal information and is something we would avoid.

I'm not sure what the best approach here would be, but the naive approach could be to extend the tap filter with a list of "tap-only headers" which will be dropped from the request. This seems straightforward, although it means that the filter would mutate the headers which I don't think it does today.