Add request and response header tags to Rack

[delner]

This pull requests allows users to define which HTTP headers appear as tags on their rack.request span.

To do so, add the following option to your configuration:

Datadog.configure do |c|
  # Add this request and response header as tags
  c.use :rack, headers: { request: ['Accept-Encoding'], response: ['Content-Encoding'] }
end

Will result in the following tags:

• http.request.headers.accept_encoding
• http.response.headers.content_encoding

By default, :headers is set to { response: ['Content-Type', 'X-Request-ID'] }.

This pull request also removes http.request_id in favor of http.request.headers.x_request_id and http.response.headers.x_request_id.

[palazzem]

I'd say to refactor the integration a bit so that it's generic and configurable for collecting headers.

[palazzem]

As we discussed, probably it's better to make it configurable as an option (easy accessible from rack and rails and sinatra) so that users can define the list of headers they want to collect (even custom ones).

Then we have to pick the right defaults, so I'd advocate to collect what we're collecting right now (e.g. request_id) and then double check with our security team for the rest.

The Agent, still has the capability to redact / obfuscate automatically collected headers and is configurable as defined in DataDog/datadog-trace-agent#398.

Sounds good @delner ?

[palazzem]

let's do it the other way around; iterate over the whitelist instead (that is smaller) instead of the headers list that is in the order of thousands of elements

[delner]

@palazzem Should we have separate lists for request and response headers? e.g. headers: { request: ['Accept-Encoding'], response: ['Content-Encoding'] }

[palazzem]

@delner I think so, probably it's way better so we differentiate a possible "clash" between request and response