apm: record only select http headers in the apm.

Don't record most HTTP request and response headers in the APM, except
for the User-Agent, Referer, Save-Data, X-Forwarded-For, Accept-Language,
and Content-Type headers. Recording every HTTP header for every request
takes up a lot of space and most of them aren't very useful.

config/initializers/00_elastic_apm.rb

@@ -1,11 +1,20 @@
-#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.config.app_name,
-#    service_version: Rails.application.config.x.git_hash,
-#  )
-#end
+ENV["ELASTIC_APM_ENABLED"] = "false" unless ENV["ELASTIC_APM_SERVER_URL"].present?
+
+Rails.application.config.to_prepare do
+  if ElasticAPM.running?
+    ElasticAPM.agent.config.service_version ||= Rails.application.config.x.git_hash
+    ElasticAPM.agent.config.service_node_name ||= ENV["NODE_NAME"]
+
+    ElasticAPM.add_filter(:transaction_filter) do |event|
+      name, _ = event.first
+
+      if name == :transaction
+        event.dig(:transaction, :context, :request, :url).except!(:protocol, :port)
+        event.dig(:transaction, :context, :request, :headers).slice!(*%w[User-Agent Referer Save-Data X-Forwarded-For Accept-Language])
+        event.dig(:transaction, :context, :response, :headers).slice!("Content-Type")
+      else
+        event
+      end
+    end
+  end
+end