metrics: fix lint warnings

This CL solve the cros lint warnings.
This is mostly due to:
* header guards
* include path
* few google style guide warnings

BUG=chromium:389229
TEST=FEATURES=test emerge-amd64-generic metrics

Change-Id: Ibbfcd2c88926bcc0c1ce9275b4ad0fb0748cd4de
Reviewed-on: https://chromium-review.googlesource.com/207248
Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
diff --git a/metrics/c_metrics_library.h b/metrics/c_metrics_library.h
index 28ae916..7f78e43 100644
--- a/metrics/c_metrics_library.h
+++ b/metrics/c_metrics_library.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef C_METRICS_LIBRARY_H_
-#define C_METRICS_LIBRARY_H_
+#ifndef METRICS_C_METRICS_LIBRARY_H_
+#define METRICS_C_METRICS_LIBRARY_H_
 
 #if defined(__cplusplus)
 extern "C" {
@@ -46,4 +46,4 @@
 #if defined(__cplusplus)
 }
 #endif
-#endif  // C_METRICS_LIBRARY_H_
+#endif  // METRICS_C_METRICS_LIBRARY_H_