Fix OpenTelemetry span processing when using logstasher

The logstasher gem monkey patches the `process_action.action_controller`
instrumentation. However the gem hasn't been updated for Rails 7, which
assumes that the payload has the 'request' object available. This
request object is used by the OpenTelemetry SDK to process spans. This
generates lots of error logs from the SDK, and prevents us from
recording some spans.

This re-monkey patches the instrumentation to use a raw_payload that is
expected in Rails 7.  Ideally, this would be fixed in the logstasher gem
however there has been little active development over the last year.
Longer term fix might be to move to a maintained logging library.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Fix OpenTelemetry errors when using with Logstasher gem ([#372](https://github.com/alphagov/govuk_app_config/pull/372))
+
 # 9.11.0
 
 * Add GDS::SSO::PermissionDeniedError to excluded exceptions list ([#366](https://github.com/alphagov/govuk_app_config/pull/366))

lib/govuk_app_config/govuk_json_logging.rb

@@ -16,6 +16,12 @@ def add_custom_fields(&block)
   end
 
   def self.configure(&block)
+    # Fixes the monkey patch from the logstasher gem to support Rails 7
+    config = Rails.application.config.logstasher
+    if (!config.controller_monkey_patch && config.controller_monkey_patch != false) || config.controller_monkey_patch == true
+      require_relative "./govuk_json_logging/rails_ext/action_controller/metal/instrumentation"
+    end
+
     configuration = Configuration.new
 
     configuration.instance_eval(&block) if block_given?

lib/govuk_app_config/govuk_json_logging/rails_ext/action_controller/metal/instrumentation.rb

@@ -0,0 +1,65 @@
+# This is copied from
+# https://github.com/shadabahmed/logstasher/blob/main/lib/logstasher/rails_ext/action_controller/metal/instrumentation.rb
+# Changes have been highlight in comments, otherwise the code is the same.
+
+module ActionController
+  module Instrumentation
+    alias_method "orig_process_action", "process_action"
+
+    def process_action(*args)
+      # The raw payload has been updated to reflect the payload structure used
+      # in Rails 7.1, primarily the addition of the `headers`, `request` keys
+      # and using `request.filtered_path` instead of `request.fullpath`.
+      # https://github.com/rails/rails/blame/d39db5d1891f7509cde2efc425c9d69bbb77e670/actionpack/lib/action_controller/metal/instrumentation.rb#L60
+      raw_payload = {
+        controller: self.class.name,
+        action: action_name,
+        request:,
+        params: request.filtered_parameters,
+        headers: request.headers,
+        format: request.format.ref,
+        method: request.request_method,
+        path: begin
+          request.filtered_path
+        rescue StandardError
+          "unknown"
+        end,
+      }
+
+      LogStasher.add_default_fields_to_payload(raw_payload, request)
+
+      LogStasher.clear_request_context
+      LogStasher.add_default_fields_to_request_context(request)
+
+      ActiveSupport::Notifications.instrument("start_processing.action_controller", raw_payload.dup)
+
+      ActiveSupport::Notifications.instrument("process_action.action_controller", raw_payload) do |payload|
+        if respond_to?(:logstasher_add_custom_fields_to_request_context)
+          logstasher_add_custom_fields_to_request_context(LogStasher.request_context)
+        end
+
+        if respond_to?(:logstasher_add_custom_fields_to_payload)
+          before_keys = raw_payload.keys.clone
+          logstasher_add_custom_fields_to_payload(raw_payload)
+          after_keys = raw_payload.keys
+          # Store all extra keys added to payload hash in payload itself. This is a thread safe way
+          LogStasher::CustomFields.add(*(after_keys - before_keys))
+        end
+
+        result = super
+
+        payload[:status] = response.status
+        append_info_to_payload(payload)
+        LogStasher.store.each do |key, value|
+          payload[key] = value
+        end
+
+        LogStasher.request_context.each do |key, value|
+          payload[key] = value
+        end
+        result
+      end
+    end
+    alias_method "logstasher_process_action", "process_action"
+  end
+end

spec/lib/govuk_json_logging_spec.rb

@@ -16,6 +16,7 @@ def self.headers
       end
     end
   end
+
   before do
     stub_const("DummyLoggingRailsApp", Class.new(Rails::Application) do
       config.hosts.clear