Initialize atomics in metrics, update style guide on atomics

src/metric.cpp

@@ -176,6 +176,7 @@ struct Metric {
         family->p->perThreadState[std::this_thread::get_id()].instances[key];
     if (!ptr) {
       ptr = std::make_unique<Counter::State>();
+      ptr->value.store(0, std::memory_order_relaxed);
     }
     Counter result;
     result.p = ptr.get();
@@ -190,6 +191,7 @@ struct Metric {
     auto &ptr = family->p->instances[key];
     if (!ptr) {
       ptr = std::make_unique<Gauge::State>();
+      ptr->value = 0.0;
     }
     Gauge result;
     result.p = ptr.get();
@@ -210,8 +212,12 @@ struct Metric {
       ptr->thresholds = family->p->buckets; // Already sorted and deduplicated
 
       // DESIGN: std::atomic is not copy-constructible
-      // Initialize vector with correct size, all atomics default to 0
+      // Initialize vector with correct size, all atomics explicitly initialized
+      // to 0
       ptr->counts = std::vector<AtomicWord>(ptr->thresholds.size());
+      for (auto &count : ptr->counts) {
+        count.store(0, std::memory_order_relaxed);
+      }
       ptr->sum.store(0, std::memory_order_relaxed);
       ptr->observations.store(0, std::memory_order_relaxed);
     }
@@ -232,8 +238,8 @@ void Counter::inc(double x) {
                  std::memory_order_relaxed);
 }
 void Gauge::inc(double x) {
-  // IMPLEMENTATION DETAIL: Gauges currently support multiple writers via mutex,
-  // though the public API documents single-writer semantics for consistency
+  // IMPLEMENTATION DETAIL: Mutex protection used internally for thread safety,
+  // but API contract remains single-writer per instance
   std::unique_lock<std::mutex> _{p->mutex};
   p->value += x;
 }

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