[NO-TICKET] Fix error during telemetry debug logging attempt

**What does this PR do?**

This PR fixes the following debug error log that would happen when
enabling debug logging + not having an agent running locally.

```
$ DD_TRACE_DEBUG=true bundle exec ruby -e "require 'datadog'; Datadog.configure {}; sleep 1"
...
DEBUG -- datadog: [datadog] (lib/datadog/core/telemetry/http/adapters/net.rb:47:in `rescue in post') Unable to send telemetry event to agent: Failed to open TCP connection to 127.0.0.1:8126 (Connection refused - connect(2) for "127.0.0.1" port 8126)
DEBUG -- datadog: [datadog] (lib/datadog/core/telemetry/emitter.rb:32:in `rescue in request') Unable to send telemetry request for event `app-started`: undefined method `code' for Datadog::Core::Telemetry::Http::InternalErrorResponse ok?: unsupported?:, not_found?:, client_error?:, server_error?:, internal_error?:true, payload:, error_type:Errno::ECONNREFUSED error:Failed to open TCP connection to 127.0.0.1:8126 (Connection refused - connect(2) for "127.0.0.1" port 8126):Datadog::Core::Telemetry::Http::InternalErrorResponse

            Datadog.logger.debug { "Telemetry sent for event `#{event.type}` (status code: #{res.code})" }
                                                                                                ^^^^^
```

Specifically, the class used to communicate errors
(`Datadog::Core::Telemetry::Http::Response::InternalErrorResponse`)
did not define a `#code` accessor, thus making the log message that
tries to debug the response code fail.

**Motivation:**

Fix this error.

**Additional Notes:**

The error itself was annoying, but harmless, as we did catch
the exception anyway.

I think the current approach of having a `Response` module that then
gets inserted into a few classes is a bit error prone.

And this is one where we definitely would've been saved by having
type signatures for this part of the code.

**How to test the change?**

I've added unit coverage + you can use the example above to
confirm it no longer happens manually.

lib/datadog/core/telemetry/emitter.rb

@@ -24,7 +24,7 @@ def request(event)
             seq_id = self.class.sequence.next
             payload = Request.build_payload(event, seq_id)
             res = @http_transport.request(request_type: event.type, payload: payload.to_json)
-            Datadog.logger.debug { "Telemetry sent for event `#{event.type}` (status code: #{res.code})" }
+            Datadog.logger.debug { "Telemetry sent for event `#{event.type}` (code: #{res.code.inspect})" }
             res
           rescue => e
             Datadog.logger.debug("Unable to send telemetry request for event `#{event.type rescue 'unknown'}`: #{e}")

lib/datadog/core/telemetry/http/response.rb

@@ -32,6 +32,10 @@ def internal_error?
             nil
           end
 
+          def code
+            nil
+          end
+
           def inspect
             "#{self.class} ok?:#{ok?} unsupported?:#{unsupported?}, " \
             "not_found?:#{not_found?}, client_error?:#{client_error?}, " \

spec/datadog/core/telemetry/emitter_spec.rb

@@ -74,6 +74,21 @@
             expect(emitter.class.sequence.instance_variable_get(:@current)).to be(original_seq_id + 1)
           end
         end
+
+        context 'when call is not successful and debug logging is enabled' do
+          let(:response) do
+            Datadog::Core::Telemetry::Http::InternalErrorResponse.new(StandardError.new('Failed call'))
+          end
+
+          it 'logs the request correctly' do
+            log_message = nil
+            expect(Datadog.logger).to receive(:debug) { |&message| log_message = message.call }
+
+            request
+
+            expect(log_message).to match('Telemetry sent for event')
+          end
+        end
       end
     end