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/persistent_integer.h b/metrics/persistent_integer.h
index 5b159c7..4a5670c 100644
--- a/metrics/persistent_integer.h
+++ b/metrics/persistent_integer.h
@@ -16,7 +16,7 @@
 
 class PersistentInteger {
  public:
-  PersistentInteger(const std::string& name);
+  explicit PersistentInteger(const std::string& name);
 
   // Virtual only because of mock.
   virtual ~PersistentInteger();
@@ -61,6 +61,6 @@
   static bool testing_;
 };
 
-}
+}  // namespace chromeos_metrics
 
 #endif  // METRICS_PERSISTENT_INTEGER_H_