Metric rack_attack_tracked_requests_total does not tracked in context of descriminator

[dsalahutdinov]

Track with discriminator does not work:

Rack::Attack.track('user agent family') do |req|
  UserAgentParser.parse(req.user_agent).family
end

Metric rack_attack_tracked_requests_total does not tagged with discriminator

[dsalahutdinov]

Here is the commit for supporting this feature here.
And the pull-request to rack-attack gem for passing a discriminator to subscribers rack/rack-attack#548