Make recording metrics never block

src/metric.hpp

@@ -73,7 +73,7 @@ template <typename T> using MetricCallback = std::function<double()>;
 // 3. When rendered, the values of all Counter objects with the same labels
 //    are summed together into a single total.
 struct Counter {
-  void inc(double = 1.0); // Increment counter (must be >= 0)
+  void inc(double = 1.0); // Increment counter (must be >= 0, never blocks)
 
 private:
   Counter();
@@ -94,9 +94,9 @@ private:
 //    are cumulative.
 // 4. For independent gauges, create them with unique labels.
 struct Gauge {
-  void inc(double = 1.0);
-  void dec(double = 1.0);
-  void set(double);
+  void inc(double = 1.0); // (never blocks)
+  void dec(double = 1.0); // (never blocks)
+  void set(double);       // (never blocks)
 
 private:
   Gauge();
@@ -116,7 +116,8 @@ private:
 // 3. When rendered, the observations from all Histogram objects with the
 //    same labels are combined into a single histogram.
 struct Histogram {
-  void observe(double); // Record observation in appropriate bucket
+  void
+  observe(double); // Record observation in appropriate bucket (never blocks)
 
 private:
   Histogram();