Allow us to force-load the Prometheus Exporter...

The Exporter currently ignores the `GOVUK_PROMETHEUS_EXPORTER`
 environment variable if it detects it is being called by rake.

As the email-alerts-service is a perpetual rake task, we need a
 way to override this for the time being.

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -1,6 +1,9 @@
 module GovukPrometheusExporter
   def self.should_configure
-    if File.basename($PROGRAM_NAME) == "rake" ||
+    # Allow us to force the Prometheus Exporter for persistent Rake tasks...
+    if ENV["GOVUK_PROMETHEUS_EXPORTER"] == "force"
+      true
+    elsif File.basename($PROGRAM_NAME) == "rake" ||
         defined?(Rails) && (Rails.const_defined?("Console") || Rails.env == "test")
       false
     else