Add Structured Event Reporter

Ref: rails#50452

This adds a Structured Event Reporter to Rails, accessible via `Rails.event`.
It allows you to report events to a subscriber, and provides mechanisms for adding tags and context to events.

Events encompass "structured logs", but also "business events", as well as telemetry events such as metrics
and logs. The Event Reporter is designed to be a single interface for producing any kind of event in a Rails
application.

We separate the emission of events from how these events reach end consumers; applications are expected to
define their own subscribers, and the Event Reporter is responsible for emitting events to these subscribers.

Author: adrianna-chang-shopify

activesupport/CHANGELOG.md

@@ -1,3 +1,44 @@
+*   Add Structured Event Reporter, accessible via `Rails.event`.
+
+    The Event Reporter provides a unified interface for producing structured events in Rails
+    applications:
+
+    ```ruby
+    Rails.event.notify("user.signup", user_id: 123, email: "[email protected]")
+    ```
+
+    It supports adding tags to events:
+
+    ```ruby
+    Rails.event.tagged("graphql") do
+      # Event includes tags: { graphql: true }
+      Rails.event.notify("user.signup", user_id: 123, email: "[email protected]")
+    end
+    ```
+
+    As well as context:
+    ```ruby
+    # All events will contain context: {request_id: "abc123", shop_id: 456}
+    Rails.event.set_context(request_id: "abc123", shop_id: 456)
+    ```
+
+    Events are emitted to subscribers. Applications register subscribers to
+    control how events are serialized and emitted. Rails provides several default
+    encoders that can be used to serialize events to common formats:
+
+    ```ruby
+    class MySubscriber
+      def emit(event)
+        encoded_event = ActiveSupport::EventReporter.encoder(:json).encode(event)
+        StructuredLogExporter.export(encoded_event)
+      end
+    end
+
+    Rails.event.subscribe(MySubscriber.new)
+    ```
+
+    *Adrianna Chang*
+
 *   Make `ActiveSupport::Logger` `#freeze`-friendly.
 
     *Joshua Young*

activesupport/lib/active_support.rb

@@ -48,6 +48,7 @@ module ActiveSupport
   autoload :ExecutionWrapper
   autoload :Executor
   autoload :ErrorReporter
+  autoload :EventReporter
   autoload :FileUpdateChecker
   autoload :EventedFileUpdateChecker
   autoload :ForkTracker
@@ -110,6 +111,9 @@ def self.eager_load!
   @error_reporter = ActiveSupport::ErrorReporter.new
   singleton_class.attr_accessor :error_reporter # :nodoc:
 
+  @event_reporter = ActiveSupport::EventReporter.new
+  singleton_class.attr_accessor :event_reporter # :nodoc:
+
   def self.cache_format_version
     Cache.format_version
   end