Set request metadata in middleware via helpers

Do not rely on the transaction's request object handling, but set the
request metadata using our helpers.

This follows our new pattern of how our instrumentation sets data on
transactions.

lib/appsignal/rack/abstract_middleware.rb

@@ -37,7 +37,7 @@ def call(env)
  Appsignal::Transaction.create(
  SecureRandom.uuid,
  Appsignal::Transaction::HTTP_REQUEST,
- request
+ Appsignal::Transaction::GenericRequest.new({})
  )
  end
 
@@ -148,6 +148,12 @@ def add_transaction_metadata_after(transaction, request)
  transaction.set_metadata("method", request_method) if request_method
 
  transaction.set_params_if_nil { params_for(request) }
+ transaction.set_session_data_if_nil do
+ request.session if request.respond_to?(:session)
+ end
+ transaction.set_headers_if_nil do
+ request.env if request.respond_to?(:env)
+ end
  queue_start = Appsignal::Rack::Utils.queue_start_from(request.env)
  transaction.set_queue_start(queue_start) if queue_start
  end