add lti variable expansion for usage metrics

why:
* internal LTI tools can key off the Canvas feature flag

refs INTEROP-7810
flag=none

test plan:
* with the flag off
* update an existing ContextExternalTool in a rails console, or install
a new tool with this variable expansion:
`metrics=$com.instructure.Account.usage_metrics_enabled`
* launch the tool
* that custom variable should have expanded to `false`
* enable the flag with
`Account.site_admin.enable_feature! :send_usage_metrics`
* launch the tool again (may require a rails cache clear)
* that custom variable should have expanded to `true`

Change-Id: I80b62acec34eeefc0be3bde30bee8e1f341b5e26
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/305701
QA-Review: Evan Battaglia <ebattaglia@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
Product-Review: Evan Battaglia <ebattaglia@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -1501,3 +1501,10 @@ in Canvas-side GET request that triggers the LTI launch.
 ```
 [{"id":"3","name":"First Module"},{"id":"5","name":"Second Module"}]
 ```
+## com.instructure.Account.usage_metrics_enabled
+
+
+**Availability**: *always*  
+
+
+

lib/lti/variable_expander.rb

@@ -1691,6 +1691,11 @@ module Lti
                          val&.to_json
                        }
 
+    register_expansion "com.instructure.Account.usage_metrics_enabled", [],
+                       lambda {
+                         @root_account.feature_enabled?(:send_usage_metrics)
+                       }
+
     private
 
     def unique_submission_dates

spec/lib/lti/variable_expander_spec.rb

@@ -464,6 +464,33 @@ module Lti
         end
       end
 
+      context "com.instructure.Account.usage_metrics_enabled" do
+        subject { variable_expander.expand_variables!(exp_hash) }
+
+        let(:exp_hash) { { test: "$com.instructure.Account.usage_metrics_enabled" } }
+
+        context "when flag is disabled" do
+          before do
+            root_account.disable_feature! :send_usage_metrics
+          end
+
+          it "expands to false" do
+            expect(subject[:test]).to eq false
+          end
+        end
+
+        context "when flag is enabled and account allows it" do
+          before do
+            root_account.settings[:enable_usage_metrics] = true
+            root_account.enable_feature! :send_usage_metrics
+          end
+
+          it "expands to true" do
+            expect(subject[:test]).to eq true
+          end
+        end
+      end
+
       it "has a substitution for com.instructure.Assignment.lti.id" do
         exp_hash = { test: "$com.instructure.Assignment.lti.id" }
         variable_expander.expand_variables!(exp_hash)