Accept exporter options (e.g. port), forward them to upstream

For now there is only `port` option, see prometheus/client_ruby#199

README.md

@@ -36,9 +36,17 @@ And then execute:
     use Yabeda::Prometheus::Exporter
     ```
 
-    Metrics will be available on `/metrics` path (configured by `:path` option).
+    Metrics will be available on `/metrics` path (configured by `:path` option), additionally metrics can be served only on specific port with `:port` option.
 
-    Also you can mount it in Rails application routes as standalone Rack application.
+    Alternatively you can mount it in Rails application routes as standalone Rack application:
+
+    ```ruby
+    Rails.application.routes.draw do
+      mount Yabeda::Prometheus::Exporter, at: "/metrics"
+    end
+    ```
+
+    Additional options (like `:port`) are also accepted and forwarded to [Prometheys Exporter](https://github.com/prometheus/client_ruby/blob/main/lib/prometheus/middleware/exporter.rb) middleware.
 
  2. Run web-server from long-running processes (delayed jobs, …):
 

lib/yabeda/prometheus/exporter.rb

@@ -14,7 +14,8 @@ class Exporter < ::Prometheus::Middleware::Exporter
       class << self
         # Allows to use middleware as standalone rack application
         def call(env)
-          @app ||= new(NOT_FOUND_HANDLER, path: "/")
+          options = env.fetch("action_dispatch.request.path_parameters", {})
+          @app ||= new(NOT_FOUND_HANDLER, path: "/", **options)
           @app.call(env)
         end
 
@@ -30,12 +31,12 @@ def start_metrics_server!(**rack_app_options)
           end
         end
 
-        def rack_app(exporter = self, path: "/metrics", logger: Logger.new(IO::NULL), use_deflater: true)
+        def rack_app(exporter = self, logger: Logger.new(IO::NULL), use_deflater: true, **exporter_options)
           ::Rack::Builder.new do
             use ::Rack::Deflater if use_deflater
             use ::Rack::CommonLogger, logger
             use ::Rack::ShowExceptions
-            use exporter, path: path
+            use exporter, **exporter_options
             run NOT_FOUND_HANDLER
           end
         end