Add more features for adding HTTP request / response headers to spans.

[rogersd]

I already have #1172 open for this, and I'll be breaking it in to smaller pieces at @lzchen 's request.

Is your feature request related to a problem?
Currently, you can only provide a list of full HTTP request / response header names to be added to the span.

There is also no capacity for header value redaction.

Describe the solution you'd like
It would be nice to be able to specify a regex or "all" to get all headers.

Header value redaction is also a must-have for us.

Describe alternatives you've considered
I considered doing this in my application, but it makes more sense to add it here.