Do not report SystemExit errors in at_exit handler (#1257)

The SystemExit can be a normal shutdown or a shutdown by the host. We do
not need to report those normal events.

If people do want to report this error they can add their own `at_exit`
handler, but let's not report it by default.

.changesets/do-not-report-systemexit-errors-from-our--at_exit--error-reporter.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: fix
+---
+
+Do not report `SystemExit` errors from our `at_exit` error reporter.

lib/appsignal/hooks/at_exit.rb

@@ -24,13 +24,23 @@ def install
       class AtExitCallback
         def self.call
           error = $! # rubocop:disable Style/SpecialGlobalVars
+          return if ignored_error?(error)
           return if Appsignal::Transaction.last_errors.include?(error)
 
           Appsignal.report_error(error) do |transaction|
             transaction.set_namespace("unhandled")
           end
           Appsignal.stop("at_exit")
         end
+
+        IGNORED_ERRORS = [
+          # Normal exits from the application we do not need to report
+          SystemExit
+        ].freeze
+
+        def self.ignored_error?(error)
+          IGNORED_ERRORS.include?(error.class)
+        end
       end
     end
   end

spec/lib/appsignal/hooks/at_exit_spec.rb

@@ -59,7 +59,7 @@ def call_callback
     end
   end
 
-  it "doesn't report the error if is also the last error reported" do
+  it "doesn't report the error if it is also the last error reported" do
     with_error(ExampleException, "error message") do |error|
       Appsignal.report_error(error)
       expect(created_transactions.count).to eq(1)
@@ -69,4 +69,15 @@ def call_callback
       end.to_not change { created_transactions.count }.from(1)
     end
   end
+
+  it "doesn't report the error if it is a SystemExit exception" do
+    with_error(SystemExit, "error message") do |error|
+      Appsignal.report_error(error)
+      expect(created_transactions.count).to eq(1)
+
+      expect do
+        call_callback
+      end.to_not change { created_transactions.count }.from(1)
+    end
+  end
 end