Add Open Telemetry initializer for Rails

This initializes Open Telemetry for Rails applications. This can be used
to send trace information to a collector. This functionality is only
enabled if the env var ENABLE_OPEN_TELEMETRY is set to "true".

README.md

@@ -104,6 +104,9 @@ end
 
 `GovukError.configure` has the same options as the Sentry client, Raven. See [the Raven docs for all configuration options](https://docs.sentry.io/clients/ruby/config).
 
+## Open Telemetry
+
+To enable Open Telemetry instrumentation for Rails set the ENABLE_OPEN_TELEMETRY="true" environment variable.
 
 ## Prometheus monitoring
 

lib/govuk_app_config.rb

@@ -3,6 +3,7 @@
 require "govuk_app_config/govuk_error"
 require "govuk_app_config/govuk_proxy/static_proxy"
 require "govuk_app_config/govuk_healthcheck"
+require "govuk_app_config/govuk_open_telemetry"
 require "govuk_app_config/govuk_prometheus_exporter"
 
 if defined?(Rails)

lib/govuk_app_config/govuk_open_telemetry.rb

@@ -0,0 +1,18 @@
+module GovukOpenTelemetry
+  def self.should_configure?
+    ENV["ENABLE_OPEN_TELEMETRY"] == "true"
+  end
+
+  def self.configure(service_name)
+    return unless should_configure?
+
+    require "opentelemetry/sdk"
+    require "opentelemetry/exporter/otlp"
+    require "opentelemetry/instrumentation/all"
+
+    OpenTelemetry::SDK.configure do |config|
+      config.service_name = service_name
+      config.use_all # enables all instrumentation!
+    end
+  end
+end

lib/govuk_app_config/railtie.rb

@@ -8,6 +8,10 @@ class Railtie < Rails::Railtie
       end
     end
 
+    initializer "govuk_app_config.configure_open_telemetry" do |app|
+      GovukOpenTelemetry.configure(app.class.module_parent_name.underscore)
+    end
+
     config.before_initialize do
       GovukLogging.configure if Rails.env.production?
     end