Pact Foundation

Hi, everyone. I am using Pact docker CLI  `verify` for verifying my provider, Pactflow is a broker. Issue is that I need to pass valid ID to graphql request variable on the fly. This situation described here: <https://docs.pact.io/provider/handling_auth#4-modify-the-request-to-use-real-credentials>.
So I have such request body: `{ "query": "....", "variables": { "ID": "placeholder" } }` , so I need to replace placeholder with real ID. I know that `--custom-middleware` option stands for this, but I am complete newb in Ruby, could you, please, provide some examples, or appropriate reading for my level, to understand the topic and be able to implement it?

After some reading, such `custom_middleware.rb` seems working:
```class AddCustomAuth &lt; Pact::ProviderVerifier::CustomMiddleware
    def initialize app
      @app = app
    end
  
    def call env
      body_string = env['rack.input'].read
      new_body_string = body_string.sub("placeholder", "realID")
      new_body_io = StringIO.new(new_body_string)
      env['rack.input'] = new_body_io
      @app.call(env)
    end
  end```
realID is going to comes from file.