Per kmixter's suggestion, install metrics headers under /usr/include/metrics.

Also, move file installation to the metrics ebuild.

Review URL: http://codereview.chromium.org/2087002
diff --git a/metrics/README b/metrics/README
index 6d011db..2c7072d 100644
--- a/metrics/README
+++ b/metrics/README
@@ -28,7 +28,7 @@
   preferred.
 
 - To access the metrics library API in the module, include the
-  <metrics_library.h> header file. The file is installed in
+  <metrics/metrics_library.h> header file. The file is installed in
   $SYSROOT/usr/include/ when the metrics library is built and
   installed.