Create a dummy MetricsReporterStub class

This handles the update_engine_sideload target, and thus avoiding
the insertion of "#ifdef _UE_SIDELOAD" everywhere when we report the
metrics for android.

Bug: 30989466
Test: build pass on marlin
Change-Id: Ib00ab8476218df15768bdbae1a3510f8584aa7c7
diff --git a/metrics_reporter_interface.h b/metrics_reporter_interface.h
index bc9d387..2c7ce5b 100644
--- a/metrics_reporter_interface.h
+++ b/metrics_reporter_interface.h
@@ -17,6 +17,8 @@
 #ifndef UPDATE_ENGINE_METRICS_REPORTER_INTERFACE_H_
 #define UPDATE_ENGINE_METRICS_REPORTER_INTERFACE_H_
 
+#include <memory>
+
 #include <base/time/time.h>
 
 #include "update_engine/common/constants.h"
@@ -29,6 +31,12 @@
 enum class ServerToCheck;
 enum class CertificateCheckResult;
 
+namespace metrics {
+
+std::unique_ptr<MetricsReporterInterface> CreateMetricsReporter();
+
+}  // namespace metrics
+
 class MetricsReporterInterface {
  public:
   virtual ~MetricsReporterInterface() = default;