Move requires to top of file

This is needed because I want to use a class from
prometheus_exporter/metric as the default for a method parameter, which
means I need to require the prometheus code before I use it.

Previously these requires happened conditionally, so if
`should_configure` was false, the prometheus code wouldn't be loaded at
all. From a quick look at the prometheus exporter code, I don't think
the requires have any side effects, so it should be fine to load them
even when we're not configuring prometheus.

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -1,3 +1,8 @@
+require "prometheus_exporter"
+require "prometheus_exporter/metric"
+require "prometheus_exporter/server"
+require "prometheus_exporter/middleware"
+
 module GovukPrometheusExporter
   def self.should_configure
     # Allow us to force the Prometheus Exporter for persistent Rake tasks...
@@ -11,13 +16,9 @@ def self.should_configure
     end
   end
 
-  def self.configure(collectors: [], default_aggregation: Prometheus::Metric::Histogram)
+  def self.configure(collectors: [], default_aggregation: PrometheusExporter::Metric::Histogram)
     return unless should_configure
 
-    require "prometheus_exporter"
-    require "prometheus_exporter/server"
-    require "prometheus_exporter/middleware"
-
     # PrometheusExporter::Metric::Histogram.DEFAULT_BUCKETS tops out at 10 but
     # we have a few controller actions which are slower than this, so we add a
     # few extra buckets for slower requests