Fix the error with accessing a non-existent attribute in the config

.rspec

@@ -0,0 +1,2 @@
+--color
+--require spec_helper

bc-prometheus-ruby.gemspec

@@ -36,6 +36,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler-audit', '>= 0.6'
   spec.add_development_dependency 'null-logger', '>= 0.1'
   spec.add_development_dependency 'pry', '>= 0.12'
+  spec.add_development_dependency 'railties', '>= 6.0.0'
   spec.add_development_dependency 'rake', '>= 10.0'
   spec.add_development_dependency 'rspec', '>= 3.8'
   spec.add_development_dependency 'rspec_junit_formatter', '>= 0.4'

lib/bigcommerce/prometheus/instrumentors/web.rb

@@ -62,7 +62,7 @@ def server
         end
 
         def setup_before_fork
-          @app.config.before_fork_callbacks = [] unless @app.config.before_fork_callbacks
+          @app.config.before_fork_callbacks = [] unless config_defined?(:before_fork_callbacks)
           @app.config.before_fork_callbacks << lambda do
             server.add_type_collector(PrometheusExporter::Server::ActiveRecordCollector.new)
             server.add_type_collector(PrometheusExporter::Server::WebCollector.new)
@@ -75,13 +75,17 @@ def setup_before_fork
         end
 
         def setup_after_fork
-          @app.config.after_fork_callbacks = [] unless @app.config.after_fork_callbacks
+          @app.config.after_fork_callbacks = [] unless config_defined?(:after_fork_callbacks)
           @app.config.after_fork_callbacks << lambda do
             ::Bigcommerce::Prometheus::Integrations::Puma.start(client: Bigcommerce::Prometheus.client)
             @collectors.each(&:start)
           end
         end
 
+        def config_defined?(config_name)
+          @app.config.respond_to?(config_name) && @app.config.public_send(config_name)
+        end
+
         def setup_middleware
           @app.middleware.unshift(PrometheusExporter::Middleware, client: Bigcommerce::Prometheus.client)
         rescue StandardError => e

spec/bigcommerce/prometheus/instrumentors/web_spec.rb

@@ -0,0 +1,40 @@
+describe Bigcommerce::Prometheus::Instrumentors::Web do
+  def run!
+    described_class.new(app: application).start
+  end
+
+  let(:application) do
+    instance_double(Rails::Application).tap do |instance|
+      allow(instance).to receive(:config).and_return(configuration)
+      allow(instance).to receive(:middleware).and_return(middleware)
+    end
+  end
+
+  let(:configuration) { Rails::Engine::Configuration.new }
+  let(:middleware) { [] }
+
+  it 'properly handles lack of the fork configs' do
+    expect(Bigcommerce::Prometheus.logger).not_to receive(:error)
+
+    run!
+
+    expect(configuration.respond_to?(:before_fork_callbacks)).to eq(true)
+    expect(configuration.respond_to?(:after_fork_callbacks)).to eq(true)
+    expect(configuration.before_fork_callbacks).to be_an_instance_of(Array)
+    expect(configuration.after_fork_callbacks).to be_an_instance_of(Array)
+  end
+
+  context 'when configuration already set' do
+    before do
+      configuration.before_fork_callbacks = [1]
+      configuration.after_fork_callbacks = [1]
+    end
+
+    it 'just adds another elements' do
+      run!
+
+      expect(configuration.before_fork_callbacks.size).to eq(2)
+      expect(configuration.after_fork_callbacks.size).to eq(2)
+    end
+  end
+end

spec/spec_helper.rb

@@ -18,6 +18,7 @@
 require_relative 'simplecov_helper'
 require 'bigcommerce/prometheus'
 require 'pry'
+require 'rails'
 Dir["#{File.join(File.dirname(__FILE__), 'support')}/**/*.rb"].sort.each { |f| require f }
 
 RSpec.configure do |config|