Store delta gauge of Garbage Collection counts (#865)

The `gc_count` metric will continuously report an growing
value, which resets on every restart or new deploy. Creating a lot of
highs and lows.

This makes it difficult to track if the app is doing more or less
garbage collection between deploys, other than tracking how steep the
increase is visually.

The delta of this metric reports the difference between probe runs,
which should result in a more stable graph that shows more heavy garbage
collection as a spike.

.changesets/report-gauge-delta-value-for-gc_count.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "change"
+---
+
+Report gauge delta value for Garbage Collection counts. This reports a more user friendly metric that doesn't always goes up until the app restarts or gets a new deploy.

lib/appsignal/probes/mri.rb

@@ -46,9 +46,18 @@ def call
           @appsignal.set_gauge("allocated_objects", allocated_objects)
         end
 
-        @appsignal.add_distribution_value("gc_count", GC.count, :metric => :gc_count)
-        @appsignal.add_distribution_value("gc_count", gc_stats[:minor_gc_count], :metric => :minor_gc_count)
-        @appsignal.add_distribution_value("gc_count", gc_stats[:major_gc_count], :metric => :major_gc_count)
+        gc_count = gauge_delta(:gc_count, GC.count)
+        if gc_count
+          @appsignal.add_distribution_value("gc_count", gc_count, :metric => :gc_count)
+        end
+        minor_gc_count = gauge_delta(:minor_gc_count, gc_stats[:minor_gc_count])
+        if minor_gc_count
+          @appsignal.add_distribution_value("gc_count", minor_gc_count, :metric => :minor_gc_count)
+        end
+        major_gc_count = gauge_delta(:major_gc_count, gc_stats[:major_gc_count])
+        if major_gc_count
+          @appsignal.add_distribution_value("gc_count", major_gc_count, :metric => :major_gc_count)
+        end
 
         @appsignal.add_distribution_value("heap_slots", gc_stats[:heap_live_slots] || gc_stats[:heap_live_slot], :metric => :heap_live)
         @appsignal.add_distribution_value("heap_slots", gc_stats[:heap_free_slots] || gc_stats[:heap_free_slot], :metric => :heap_free)

spec/lib/appsignal/probes/mri_spec.rb

@@ -49,11 +49,17 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
         expect_gauge_value("gc_total_time")
       end
 
-      it "tracks GC runs" do
+      it "tracks GC run count" do
+        expect(GC).to receive(:count).and_return(10, 15)
+        expect(GC).to receive(:stat).and_return(
+          { :minor_gc_count => 10, :major_gc_count => 10 },
+          :minor_gc_count => 16, :major_gc_count => 17
+        )
         probe.call
-        expect_distribution_value("gc_count", :gc_count)
-        expect_distribution_value("gc_count", :major_gc_count)
-        expect_distribution_value("gc_count", :minor_gc_count)
+        probe.call
+        expect_distribution_value("gc_count", :gc_count, 5)
+        expect_distribution_value("gc_count", :minor_gc_count, 6)
+        expect_distribution_value("gc_count", :major_gc_count, 7)
       end
 
       it "tracks object allocation" do
@@ -75,11 +81,15 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
     end
   end
 
-  def expect_distribution_value(expected_key, metric)
+  def expect_distribution_value(expected_key, metric, expected_value = nil)
     expect(appsignal_mock.distribution_values).to satisfy do |distribution_values|
       distribution_values.any? do |distribution_value|
         key, value, metadata = distribution_value
-        key == expected_key && !value.nil? && metadata == { :metric => metric }
+        next unless key == expected_key
+        next unless expected_value ? expected_value == value : !value.nil?
+        next unless metadata == { :metric => metric }
+
+        true
       end
     end
   end