newrelic: fix newrelic starting without license key.

Fix an issue where the New Relic agent always started in the production
environment, even when a license key wasn't configured.

Also make the New Relic agent log to stdout instead of log/newrelic_agent.log.

config/danbooru_default_config.rb

@@ -581,6 +581,12 @@ module Danbooru
     def redis_url
       # "redis://localhost:6379"
     end
+
+    # Optional. The license key for your New Relic account.
+    # https://newrelic.com/
+    # https://docs.newrelic.com/docs/accounts/accounts-billing/account-setup/new-relic-license-key/
+    def new_relic_license_key
+    end
   end
 
   EnvironmentConfiguration = Struct.new(:config) do

config/initializers/00_newrelic.rb

@@ -0,0 +1,14 @@
+if Danbooru.config.new_relic_license_key.present?
+  require "new_relic/control"
+
+  # https://github.com/newrelic/newrelic-ruby-agent/blob/1ef4082fe97fd19aeccb3f392a44ef3becae66d3/lib/newrelic_rpm.rb#L40
+  # https://github.com/newrelic/newrelic-ruby-agent/blob/1ef4082fe97fd19aeccb3f392a44ef3becae66d3/lib/new_relic/agent.rb#L349
+  NewRelic::Control.instance.init_plugin(
+    app_name: Danbooru.config.canonical_app_name,
+    license_key: Danbooru.config.new_relic_license_key,
+    log_level: Danbooru.config.debug_mode ? "debug" : "error",
+    log: Rails.logger,
+    "rake.tasks": ["maintenance:.*"],
+    config: Rails.application.config,
+  )
+end

config/initializers/unicorn.rb

@@ -1,7 +0,0 @@
-# Ensure the agent is started using Unicorn.
-# This is needed when using Unicorn and preload_app is not set to true.
-# See https://newrelic.com/docs/ruby/no-data-with-unicorn
-if defined? ::NewRelic
-  ::NewRelic::Agent.manual_start
-  ::NewRelic::Agent.after_fork(:force_reconnect => true)
-end