metricsd: move timer.h into include/metrics/

Move timer.h into the include/metrics/ folder since some
source files in shill need to include this header file.
Adjust the timer.h include paths in the metricsd accordingly.

BUG: None
Change-Id: Id2f5e079d3a095f40d0f4d4e0d438bd22f1274ff
TEST: "mma" in metricsd folder succeeds.
diff --git a/metricsd/timer.cc b/metricsd/timer.cc
index 7b00cc0..0c2c119 100644
--- a/metricsd/timer.cc
+++ b/metricsd/timer.cc
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-#include "timer.h"
+#include "metrics/timer.h"
 
 #include <string>