instrumentation.rb

# config/initializers/instrumentation.rb

# Subscribe to grape request and log with Rails.logger
ActiveSupport::Notifications.subscribe('grape.request') do |name, starts, ends, notification_id, payload|
  Rails.logger.info '[API] %s %s (%.3f ms) -> %s %s%s' % [
    payload[:request_method],
    payload[:request_path],
    (ends-starts)*1000,
    (payload[:response_status] || "error"),
    payload[:x_organization] ? "| X-Org: #{payload[:x_organization]}" : "",
    payload[:params] ? "| #{payload[:params].inspect}" : ""
  ]
end

logger.rb

# app/controllers/api/logger.rb

class API::Logger
  def initialize(app)
    @app = app
  end

  def call(env)
    payload = {
      remote_addr:    env['REMOTE_ADDR'],
      request_method: env['REQUEST_METHOD'],
      request_path:   env['PATH_INFO'],
      request_query:  env['QUERY_STRING'],
      x_organization: env['HTTP_X_ORGANIZATION']
    }

    ActiveSupport::Notifications.instrument "grape.request", payload do
      @app.call(env).tap do |response|
        payload[:params] = env["api.endpoint"].params.to_hash
        payload[:params].delete("route_info")
        payload[:params].delete("format")
        payload[:response_status] = response[0]
      end
    end
  end
end

Just for information, we used your idea in this gem : https://github.com/aserafin/grape_logging ;)

Thanks for the idea !