Initialize atomics in metrics, update style guide on atomics

src/metric.hpp

@@ -58,6 +58,7 @@ private:
 // THREAD SAFETY: Each gauge instance has exactly ONE writer thread (the one
 // that created it). It is an error to call inc()/dec()/set() from any thread
 // other than the creating thread.
+// IMPLEMENTATION NOTE: Mutex protection is an internal implementation detail.
 struct Gauge {
   void inc(double = 1.0); // Increase gauge value - SINGLE WRITER ONLY
   void dec(double = 1.0); // Decrease gauge value - SINGLE WRITER ONLY