Move ScoutApm instrumentation into an initializer

edd6330 added ScoutApm instrumentation to methods on the following:
- ReactOnRails::Helper
- ReactOnRails::ServerRenderingPool::RubyEmbeddedJavaScript

Previously, this instrumentation was installed at gem load time using
`defined?(ScoutApm)` guards. This caused two problems:

1. Gem ordering dependency: if scout_apm appeared after react_on_rails
   in the Gemfile, ScoutApm was not yet defined when react_on_rails
   loaded, so instrumentation was silently skipped.

2. Noisy log output: if scout_apm appeared before react_on_rails,
   instrumentation was installed before ScoutApm was initialized (via
   its "scout_apm.start" Railtie initializer), causing INFO-level
   messages to be written to stdout during boot.

This commit moves the instrumentation into ReactOnRails::Engine with
an initializer explicitly configured to run after ScoutApm initializes
("scout_apm.start"). This ensures instrumentation is properly installed
regardless of gem ordering and only after ScoutApm is fully configured.

Author: tonyta

CHANGELOG.md

@@ -29,6 +29,7 @@ Changes since the last non-beta release.
 
 #### Fixed
 
+- **Fixed ScoutApm instrumentation depending on Gemfile ordering**. ScoutApm instrumentation for `react_component`, `react_component_hash`, and `exec_server_render_js` was previously installed at gem load time using `defined?(ScoutApm)` guards, which meant it was silently skipped if `scout_apm` appeared after `react_on_rails` in the Gemfile, and produced noisy INFO log messages if it appeared before (since ScoutApm wasn't yet initialized). Moved instrumentation into an initializer that runs after `scout_apm.start`, ensuring it works regardless of gem ordering and only after ScoutApm is fully configured. [PR 2442](https://github.com/shakacode/react_on_rails/pull/2442) by [tonyta](https://github.com/tonyta).
 - **Fixed string values interpolated into generated JS code without proper escaping**. All string values (component names, DOM IDs, Redux store names) embedded in server-rendering JavaScript now use `.to_json` instead of unescaped single-quoted interpolation, preventing potential JS breakage from special characters. [PR 2440](https://github.com/shakacode/react_on_rails/pull/2440) by [AbanoubGhadban](https://github.com/AbanoubGhadban).
 
 #### Pro

react_on_rails/lib/react_on_rails/engine.rb

@@ -83,6 +83,23 @@ def self.package_json_missing?
       !File.exist?(VersionChecker::NodePackageVersion.package_json_path)
     end
 
+    # Install ScoutApm instrumentation after ScoutApm is configured via "scout_apm.start" initializer.
+    # https://github.com/scoutapp/scout_apm_ruby/blob/v6.1.0/lib/scout_apm.rb#L221
+    initializer "react_on_rails.scout_apm_instrumentation", after: "scout_apm.start" do
+      next unless defined?(ScoutApm)
+
+      ReactOnRails::Helper.class_eval do
+        include ScoutApm::Tracer
+        instrument_method :react_component, type: "ReactOnRails", name: "react_component"
+        instrument_method :react_component_hash, type: "ReactOnRails", name: "react_component_hash"
+      end
+
+      ReactOnRails::ServerRenderingPool::RubyEmbeddedJavaScript.singleton_class.class_eval do
+        include ScoutApm::Tracer
+        instrument_method :exec_server_render_js, type: "ReactOnRails", name: "ExecJs React Server Rendering"
+      end
+    end
+
     config.to_prepare do
       ReactOnRails::ServerRenderingPool.reset_pool
     end

react_on_rails/lib/react_on_rails/helper.rb

@@ -714,13 +714,6 @@ def in_mailer?
       controller.is_a?(ActionMailer::Base)
     end
 
-    if defined?(ScoutApm)
-      include ScoutApm::Tracer
-
-      instrument_method :react_component, type: "ReactOnRails", name: "react_component"
-      instrument_method :react_component_hash, type: "ReactOnRails", name: "react_component_hash"
-    end
-
     def raise_missing_autoloaded_bundle(react_component_name)
       raise ReactOnRails::SmartError.new(
         error_type: :missing_auto_loaded_bundle,

react_on_rails/lib/react_on_rails/server_rendering_pool/ruby_embedded_java_script.rb

@@ -211,12 +211,6 @@ def console_polyfill
           JS
         end
 
-        if defined?(ScoutApm)
-          include ScoutApm::Tracer
-
-          instrument_method :exec_server_render_js, type: "ReactOnRails", name: "ExecJs React Server Rendering"
-        end
-
         private
 
         def file_url_to_string(url)

react_on_rails/spec/react_on_rails/engine_spec.rb

@@ -166,6 +166,91 @@ module ReactOnRails
       end
     end
 
+    describe "ScoutApm instrumentation initializer" do
+      subject(:initializer) { described_class.initializers.find { |i| i.name.include?("scout_apm") } }
+
+      it "defines a named Rails initializer to run after scout_apm.start" do
+        expect(initializer.name).to eq "react_on_rails.scout_apm_instrumentation"
+        expect(initializer.after).to eq "scout_apm.start"
+      end
+
+      describe "react_on_rails.scout_apm_instrumentation" do
+        let(:mock_scout_tracer) do
+          #
+          # Simplified mock of ScoutApm::Tracer that mirrors its real implementation.
+          # https://github.com/scoutapp/scout_apm_ruby/blob/v6.1.0/lib/scout_apm/tracer.rb#L47-L70
+          #
+          Module.new do
+            def self.included(base)
+              base.define_singleton_method(:instrument_method) do |method_name, **|
+                raise "method does not exist: #{method_name}" unless method_defined?(method_name)
+
+                instrumented_name = :"#{method_name}_with_test_instrument"
+                uninstrumented_name = :"#{method_name}_without_test_instrument"
+
+                define_method(instrumented_name) do |*args, **kwargs, &block|
+                  send(uninstrumented_name, *args, **kwargs, &block)
+                end
+
+                alias_method uninstrumented_name, method_name
+                alias_method method_name, instrumented_name
+              end
+            end
+          end
+        end
+
+        let(:mock_helper) { Module.new.include(ReactOnRails::Helper) }
+        let(:mock_rb_embedded_js) { Class.new(ReactOnRails::ServerRenderingPool::RubyEmbeddedJavaScript) }
+
+        before do
+          stub_const("ReactOnRails::Helper", mock_helper)
+          stub_const("ReactOnRails::ServerRenderingPool::RubyEmbeddedJavaScript", mock_rb_embedded_js)
+        end
+
+        context "when ScoutApm is not defined" do
+          it "does not instrument Helper#react_component" do
+            initializer.run
+            expect(mock_helper.instance_methods(false)).not_to include(:react_component_with_test_instrument)
+            expect(mock_helper.instance_methods(false)).not_to include(:react_component_without_test_instrument)
+          end
+
+          it "does not instrument Helper#react_component_hash" do
+            initializer.run
+            expect(mock_helper.instance_methods(false)).not_to include(:react_component_hash_with_test_instrument)
+            expect(mock_helper.instance_methods(false)).not_to include(:react_component_hash_without_test_instrument)
+          end
+
+          it "does not instrument RubyEmbeddedJavaScript.exec_server_render_js" do
+            initializer.run
+            expect(mock_rb_embedded_js.methods(false)).not_to include(:exec_server_render_js_with_test_instrument)
+            expect(mock_rb_embedded_js.methods(false)).not_to include(:exec_server_render_js_without_test_instrument)
+          end
+        end
+
+        context "when ScoutApm is defined" do
+          before { stub_const("ScoutApm::Tracer", mock_scout_tracer) }
+
+          it "instruments Helper#react_component" do
+            initializer.run
+            expect(mock_helper.instance_methods(false)).to include(:react_component_with_test_instrument)
+            expect(mock_helper.instance_methods(false)).to include(:react_component_without_test_instrument)
+          end
+
+          it "instruments Helper#react_component_hash" do
+            initializer.run
+            expect(mock_helper.instance_methods(false)).to include(:react_component_hash_with_test_instrument)
+            expect(mock_helper.instance_methods(false)).to include(:react_component_hash_without_test_instrument)
+          end
+
+          it "instruments RubyEmbeddedJavaScript.exec_server_render_js" do
+            initializer.run
+            expect(mock_rb_embedded_js.methods(false)).to include(:exec_server_render_js_with_test_instrument)
+            expect(mock_rb_embedded_js.methods(false)).to include(:exec_server_render_js_without_test_instrument)
+          end
+        end
+      end
+    end
+
     describe "automatic rake task loading" do
       # Rails::Engine automatically loads all .rake files from lib/tasks/
       # This test verifies that our rake tasks are loaded without needing