Only report gc_time metric if profiler is enabled

.changesets/support-temporarily-disabling-gc-profiling.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "add"
+---
+
+Support temporarily disabling GC profiling without reporting inaccurate `gc_time` metric durations. The MRI probe's `gc_time` will not report any value when the `GC::Profiler.enabled?` returns `false`.

lib/appsignal/probes/mri.rb

@@ -31,8 +31,10 @@ def call
         )
 
         set_gauge("thread_count", Thread.list.size)
-        gauge_delta(:gc_time, @gc_profiler.total_time) do |gc_time|
-          set_gauge("gc_time", gc_time) if gc_time > 0
+        if GC::Profiler.enabled?
+          gauge_delta(:gc_time, @gc_profiler.total_time) do |gc_time|
+            set_gauge("gc_time", gc_time) if gc_time > 0
+          end
         end
 
         gc_stats = GC.stat

spec/lib/appsignal/probes/mri_spec.rb

@@ -39,6 +39,7 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
       let(:hostname) { nil }
       before do
         allow(gc_profiler_mock).to receive(:total_time)
+        allow(GC::Profiler).to receive(:enabled?).and_return(true)
       end
 
       it "should track vm metrics" do
@@ -70,6 +71,40 @@ def set_gauge(*args) # rubocop:disable Naming/AccessorMethodName
         end
       end
 
+      context "when GC profiling is disabled" do
+        it "does not report a gc_time metric" do
+          allow(GC::Profiler).to receive(:enabled?).and_return(false)
+          expect(gc_profiler_mock).to_not receive(:total_time)
+          probe.call # Normal call, create a cache
+          probe.call # Report delta value based on cached value
+          metrics = appsignal_mock.gauges.map { |(key)| key }
+          expect(metrics).to_not include("gc_time")
+        end
+
+        it "does not report a gc_time metric while disable" do
+          # While enabled
+          allow(GC::Profiler).to receive(:enabled?).and_return(true)
+          expect(gc_profiler_mock).to receive(:total_time).and_return(10, 15)
+          probe.call # Normal call, create a cache
+          probe.call # Report delta value based on cached value
+          expect_gauges([["gc_time", 5]])
+
+          # While disabled
+          allow(GC::Profiler).to receive(:enabled?).and_return(false)
+          probe.call # Call twice to make sure any caches resets wouldn't mess up the assertion
+          probe.call
+          # Does not include any newly reported metrics
+          expect_gauges([["gc_time", 5]])
+
+          # When enabled after being disabled for a while, it only reports the
+          # newly reported time since it was renabled
+          allow(GC::Profiler).to receive(:enabled?).and_return(true)
+          expect(gc_profiler_mock).to receive(:total_time).and_return(25)
+          probe.call
+          expect_gauges([["gc_time", 5], ["gc_time", 10]])
+        end
+      end
+
       it "tracks GC run count" do
         expect(GC).to receive(:count).and_return(10, 15)
         expect(GC).to receive(:stat).and_return(