Avoid noexcept error when using time_point with atomic

Author: ljbade

core/include/prometheus/gauge.h

@@ -75,8 +75,8 @@ class PROMETHEUS_CPP_CORE_EXPORT Gauge {
  private:
   void Change(double);
   std::atomic<double> value_{0.0};
-  std::atomic<std::chrono::system_clock::time_point> time_{
-      std::chrono::system_clock::now()};
+  std::atomic<std::chrono::system_clock::duration> time_{
+      std::chrono::system_clock::now().time_since_epoch()};
 };
 
 /// \brief Return a builder to configure and register a Gauge metric.

core/src/gauge.cc

@@ -16,7 +16,7 @@ void Gauge::Decrement(const double value) { Change(-1.0 * value); }
 
 void Gauge::Set(const double value) {
   value_.store(value);
-  time_.store(std::chrono::system_clock::now());
+  time_.store(std::chrono::system_clock::now().time_since_epoch());
 }
 
 void Gauge::Change(const double value) {
@@ -25,7 +25,7 @@ void Gauge::Change(const double value) {
   while (!value_.compare_exchange_weak(current, current + value)) {
     // intentionally empty block
   }
-  time_.store(std::chrono::system_clock::now());
+  time_.store(std::chrono::system_clock::now().time_since_epoch());
 }
 
 void Gauge::SetToCurrentTime() {
@@ -43,8 +43,8 @@ ClientMetric Gauge::Collect() const {
 
 bool Gauge::Expired(const std::chrono::system_clock::time_point& time,
                     const std::chrono::seconds& ttl) const {
-  return std::chrono::duration_cast<std::chrono::seconds>(time -
-                                                          time_.load()) >= ttl;
+  return std::chrono::duration_cast<std::chrono::seconds>(
+             time.time_since_epoch() - time_.load()) >= ttl;
 }
 
 }  // namespace prometheus