Fix 'uninitialized constant GRPC::Interceptor' when with the 'gapic-c…

…ommon' gem

lib/datadog/tracing/contrib/grpc/integration.rb

@@ -20,7 +20,14 @@ def self.version
           end
 
           def self.loaded?
-            !defined?(::GRPC).nil?
+            !defined?(::GRPC).nil? &&
+              # When using the Google "Calendar User Availability API"
+              # (https://developers.google.com/calendar/api/user-availability/reference/rest), though the gem
+              # `google-cloud-calendar-useravailability-v1alpha` (currently in private preview),
+              # it's possible to load `GRPC` without loading the rest of the `grpc` gem. See:
+              # https://github.com/googleapis/gapic-generator-ruby/blob/f1c2e73219453e497b6ec2dc807a907e939e1342/gapic-common/lib/gapic/common.rb#L15-L16
+              # When this happens, there are no gRPC components of interest to instrument.
+              !defined?(::GRPC::Interceptor).nil? && !defined?(::GRPC::InterceptionContext).nil?
           end
 
           def self.compatible?

spec/datadog/tracing/contrib/grpc/integration_spec.rb

@@ -28,7 +28,26 @@
     context 'when GRPC is defined' do
       before { stub_const('GRPC', Class.new) }
 
-      it { is_expected.to be true }
+      context 'when GRPC::Interceptor is defined' do
+        before { stub_const('GRPC::Interceptor', Class.new) }
+
+        context 'when GRPC::InterceptionContext is defined' do
+          before { stub_const('GRPC::InterceptionContext', Class.new) }
+          it { is_expected.to be true }
+        end
+
+        context 'when GRPC::InterceptionContext is not defined' do
+          before { hide_const('GRPC::InterceptionContext') }
+
+          it { is_expected.to be false }
+        end
+      end
+
+      context 'when GRPC::Interceptor is not defined' do
+        before { hide_const('GRPC::Interceptor') }
+
+        it { is_expected.to be false }
+      end
     end
 
     context 'when GRPC is not defined' do