add C wrapper for libmetrics

TEST=unit tests + build&test crosmetrics plugin for flimflam

Review URL: http://codereview.chromium.org/2832008
diff --git a/metrics/metrics_library.h b/metrics/metrics_library.h
index a9ac0bb..80bea70 100644
--- a/metrics/metrics_library.h
+++ b/metrics/metrics_library.h
@@ -56,6 +56,7 @@
   static bool SendToAutotest(const std::string& name, int value);
 
  private:
+  friend class CMetricsLibraryTest;
   friend class MetricsLibraryTest;
   FRIEND_TEST(MetricsLibraryTest, FormatChromeMessage);
   FRIEND_TEST(MetricsLibraryTest, FormatChromeMessageTooLong);