Make prometheus service querying approach much nicer wrt to arity and default function params

app/models/project_services/prometheus_service.rb

@@ -91,16 +91,17 @@ class PrometheusService < MonitoringService
   end
   def matched_metrics
-    with_reactive_cache(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name, nil, &:itself)
+    with_reactive_cache(Gitlab::Prometheus::Queries::MatchedMetricsQuery.name, &:itself)
   end
   # Cache metrics for specific environment
-  def calculate_reactive_cache(query_class_name, environment_id, *args)
+  def calculate_reactive_cache(query_class_name, *args)
     return unless active? && project && !project.pending_delete?
+    environment_id = args.first
     client = client(environment_id)
-    data = Kernel.const_get(query_class_name).new(client).query(environment_id, *args)
+    data = Kernel.const_get(query_class_name).new(client).query(*args)
     {
       success: true,
       data: data,

lib/gitlab/prometheus/queries/matched_metrics_query.rb

@@ -4,7 +4,7 @@ module Gitlab
       class MatchedMetricsQuery < BaseQuery
         MAX_QUERY_ITEMS = 40.freeze
-        def query(_ = nil)
+        def query
           groups_data.map do |group, data|
             {
               group: group.name,

spec/models/project_services/prometheus_service_spec.rb

@@ -75,6 +75,25 @@ describe PrometheusService, :use_clean_rails_memory_store_caching do
     end
   end
+  describe '#matched_metrics' do
+    let(:matched_metrics_query) { Gitlab::Prometheus::Queries::MatchedMetricsQuery }
+    let(:client) { double(:client, label_values: nil) }
+
+    context 'with valid data' do
+      subject { service.matched_metrics }
+
+      before do
+        allow(service).to receive(:client).and_return(client)
+        synchronous_reactive_cache(service)
+      end
+
+      it 'returns reactive data' do
+        expect(subject[:success]).to be_truthy
+        expect(subject[:data]).to eq([])
+      end
+    end
+  end
+
   describe '#deployment_metrics' do
     let(:deployment) { build_stubbed(:deployment) }
     let(:deployment_query) { Gitlab::Prometheus::Queries::DeploymentQuery }

spec/support/reactive_caching_helpers.rb

@@ -13,6 +13,12 @@ module ReactiveCachingHelpers
     write_reactive_cache(subject, data, *qualifiers) if data
   end
+  def synchronous_reactive_cache(subject)
+    allow(service).to receive(:with_reactive_cache) do |*args, &block|
+      block.call(service.calculate_reactive_cache(*args))
+    end
+  end
+
   def read_reactive_cache(subject, *qualifiers)
     Rails.cache.read(reactive_cache_key(subject, *qualifiers))
   end