Only allow Gauges to use :most_recent aggregation

Using this aggregation with any other metric type is almost certainly
not what the user intended, and it'll result in counters that go up
and down, and completely inconsistent histograms.

Signed-off-by: Daniel Magliola <danielmagliola@gocardless.com>

Author: Daniel Magliola

lib/prometheus/client/data_stores/direct_file_store.rb

@@ -45,7 +45,7 @@ def for_metric(metric_name, metric_type:, metric_settings: {})
           end
 
           settings = default_settings.merge(metric_settings)
-          validate_metric_settings(settings)
+          validate_metric_settings(metric_type, settings)
 
           MetricStore.new(metric_name: metric_name,
                           store_settings: @store_settings,
@@ -54,7 +54,7 @@ def for_metric(metric_name, metric_type:, metric_settings: {})
 
         private
 
-        def validate_metric_settings(metric_settings)
+        def validate_metric_settings(metric_type, metric_settings)
           unless metric_settings.has_key?(:aggregation) &&
             AGGREGATION_MODES.include?(metric_settings[:aggregation])
             raise InvalidStoreSettingsError,
@@ -65,6 +65,11 @@ def validate_metric_settings(metric_settings)
             raise InvalidStoreSettingsError,
                   "Only :aggregation setting can be specified"
           end
+
+          if metric_settings[:aggregation] == MOST_RECENT && metric_type != :gauge
+            raise InvalidStoreSettingsError,
+                  "Only :gauge metrics support :most_recent aggregation"
+          end
         end
 
         class MetricStore

spec/prometheus/client/data_stores/direct_file_store_spec.rb

@@ -14,7 +14,7 @@
 
   it_behaves_like Prometheus::Client::DataStores
 
-  it "only accepts valid :aggregation as Metric Settings" do
+  it "only accepts valid :aggregation values as Metric Settings" do
     expect do
       subject.for_metric(:metric_name,
                          metric_type: :counter,
@@ -26,14 +26,43 @@
                          metric_type: :counter,
                          metric_settings: { aggregation: :invalid })
     end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
+  end
 
+  it "only accepts valid keys as Metric Settings" do
+    # the only valid key at the moment is :aggregation
     expect do
       subject.for_metric(:metric_name,
                          metric_type: :counter,
                          metric_settings: { some_setting: true })
     end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
   end
 
+  it "only accepts :most_recent aggregation for gauges" do
+    expect do
+      subject.for_metric(:metric_name,
+                         metric_type: :gauge,
+                         metric_settings: { aggregation: Prometheus::Client::DataStores::DirectFileStore::MOST_RECENT })
+    end.not_to raise_error
+
+    expect do
+      subject.for_metric(:metric_name,
+                         metric_type: :counter,
+                         metric_settings: { aggregation: Prometheus::Client::DataStores::DirectFileStore::MOST_RECENT })
+    end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
+
+    expect do
+      subject.for_metric(:metric_name,
+                         metric_type: :histogram,
+                         metric_settings: { aggregation: Prometheus::Client::DataStores::DirectFileStore::MOST_RECENT })
+    end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
+
+    expect do
+      subject.for_metric(:metric_name,
+                         metric_type: :summary,
+                         metric_settings: { aggregation: Prometheus::Client::DataStores::DirectFileStore::MOST_RECENT })
+    end.to raise_error(Prometheus::Client::DataStores::DirectFileStore::InvalidStoreSettingsError)
+  end
+
   it "raises when aggregating if we get to that that point with an invalid aggregation mode" do
     # This is basically just for coverage of a safety clause that can never be reached
     allow(subject).to receive(:validate_metric_settings) # turn off validation