Coverage improvement and concern extraction for rate limit headers in API controller (#3625)

* Coverage for rate limit headers

* Move rate limit headers methods to concern

* Move throttle check to condition on before_action

* Move match_data variable into method

* Move utc timestamp to separate method

* Move header setting into smaller methods

* specs cleanup

app/controllers/concerns/rate_limit_headers.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RateLimitHeaders
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :set_rate_limit_headers, if: :rate_limited_request?
+  end
+
+  private
+
+  def set_rate_limit_headers
+    apply_header_limit
+    apply_header_remaining
+    apply_header_reset
+  end
+
+  def rate_limited_request?
+    !request.env['rack.attack.throttle_data'].nil?
+  end
+
+  def apply_header_limit
+    response.headers['X-RateLimit-Limit'] = rate_limit_limit
+  end
+
+  def rate_limit_limit
+    api_throttle_data[:limit].to_s
+  end
+
+  def apply_header_remaining
+    response.headers['X-RateLimit-Remaining'] = rate_limit_remaining
+  end
+
+  def rate_limit_remaining
+    (api_throttle_data[:limit] - api_throttle_data[:count]).to_s
+  end
+
+  def apply_header_reset
+    response.headers['X-RateLimit-Reset'] = rate_limit_reset
+  end
+
+  def rate_limit_reset
+    (request_time + reset_period_offset).iso8601(6)
+  end
+
+  def api_throttle_data
+    request.env['rack.attack.throttle_data']['api']
+  end
+
+  def request_time
+    @_request_time ||= Time.now.utc
+  end
+
+  def reset_period_offset
+    api_throttle_data[:period] - request_time.to_i % api_throttle_data[:period]
+  end
+end