Require sidekiq/api in GovukPrometheusExporter

The workers were erroring with:
```
PrometheusExporter::Instrumentation::SidekiqProcess Prometheus Exporter Failed
To Collect Stats uninitialized constant Sidekiq::Process
PrometheusExporter::Instrumentation::SidekiqQueue Prometheus Exporter Failed To
Collect Stats uninitialized constant Sidekiq::Queue
PrometheusExporter::Instrumentation::SidekiqStats Prometheus Exporter Failed To
Collect Stats uninitialized constant Sidekiq::Stats
```

and failing to send metrics following a switch from Statsd to Prometheus Exporter:
alphagov/govuk_sidekiq@763144d

Sidekiq::Process, Sidekiq::Queue, Sidekiq::Stats are defined in Sidekiq API. It
appears that to get access to the functionality the API needs to be required
See https://github.com/sidekiq/sidekiq/wiki/API

lib/govuk_app_config/govuk_prometheus_exporter.rb

@@ -20,6 +20,7 @@ def self.configure
 
     if defined?(Sidekiq)
       Sidekiq.configure_server do |config|
+        require "sidekiq/api"
         require "prometheus_exporter/instrumentation"
         config.server_middleware do |chain|
           chain.add PrometheusExporter::Instrumentation::Sidekiq