Some code improvements

- Use strong_memoize for prometheus_adapter since it can be nil in some
cases.
- Do not phrase spec descriptions with 'should'.

app/services/prometheus/proxy_service.rb

@@ -3,6 +3,7 @@
 module Prometheus
   class ProxyService < BaseService
     include ReactiveCaching
+    include Gitlab::Utils::StrongMemoize
 
     self.reactive_cache_key = ->(service) { service.cache_key }
     self.reactive_cache_lease_timeout = 30.seconds
@@ -91,7 +92,9 @@ module Prometheus
     end
 
     def prometheus_adapter
-      @prometheus_adapter ||= @proxyable.prometheus_adapter
+      strong_memoize(:prometheus_adapter) do
+        @proxyable.prometheus_adapter
+      end
     end
 
     def prometheus_client_wrapper

spec/services/prometheus/proxy_service_spec.rb

@@ -48,7 +48,7 @@ describe Prometheus::ProxyService do
         allow(environment).to receive(:prometheus_adapter).and_return(nil)
       end
 
-      it 'should return error' do
+      it 'returns error' do
         expect(subject.execute).to eq(
           status: :error,
           message: 'No prometheus server found',
@@ -63,7 +63,7 @@ describe Prometheus::ProxyService do
         allow(prometheus_adapter).to receive(:can_query?).and_return(false)
       end
 
-      it 'should return error' do
+      it 'returns error' do
         expect(subject.execute).to eq(
           status: :error,
           message: 'No prometheus server found',