Fix recursive call with prometheus_exporter and opentelemetry instrum…

…entation

This switches prometheus_exporter's method patching to use `prepend`
instead of `alias_method`. Prepend is negligibly slower than
alias_method, but is more compatible other libraries.

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -40,7 +40,7 @@ def self.configure
       server.start
 
       if defined?(Rails)
-        Rails.application.middleware.unshift PrometheusExporter::Middleware
+        Rails.application.middleware.unshift PrometheusExporter::Middleware, instrument: :prepend
       end
 
       if defined?(Sinatra)