Add Elastic APM integration.

https://www.elastic.co/guide/en/apm/agent/ruby/4.x/introduction.html

config/danbooru_default_config.rb

@@ -547,6 +547,13 @@ module Danbooru
       # "redis://localhost:6379"
     end
 
+    # Optional. The URL of the Elastic APM server. Used for application performance monitoring.
+    #
+    # https://www.elastic.co/observability/application-performance-monitoring
+    def elastic_apm_server_url
+      # "http://localhost:8200"
+    end
+
     # True if the Winter Sale is active.
     def is_promotion?
       false

config/initializers/00_elastic_apm.rb

@@ -0,0 +1,10 @@
+if Danbooru.config.elastic_apm_server_url.present?
+  require "elastic_apm"
+
+  # https://www.elastic.co/guide/en/apm/agent/ruby/4.x/api.html#api-agent-start
+  # https://www.elastic.co/guide/en/apm/agent/ruby/4.x/configuration.html
+  ElasticAPM::Rails.start(
+    server_url: Danbooru.config.elastic_apm_server_url,
+    service_name: "Danbooru",
+  )
+end