Metrics system scaffold

src/metric.hpp

@@ -0,0 +1,63 @@
+#pragma once
+
+#include "arena_allocator.hpp"
+#include <initializer_list>
+#include <span>
+#include <string_view>
+#include <type_traits>
+
+namespace metric {
+
+struct Counter {
+  void inc(double = 1.0);
+
+private:
+  Counter();
+  friend struct Metric;
+  struct State;
+  State *p;
+};
+
+struct Gauge {
+  void inc(double = 1.0);
+  void dec(double = 1.0);
+  void set(double);
+
+private:
+  Gauge();
+  friend struct Metric;
+  struct State;
+  State *p;
+};
+
+struct Histogram {
+  void observe(double);
+
+private:
+  Histogram();
+  friend struct Metric;
+  struct State;
+  State *p;
+};
+
+template <class T> struct Family {
+  static_assert(std::is_same_v<T, Counter> || std::is_same_v<T, Gauge> ||
+                std::is_same_v<T, Histogram>);
+  T create(
+      std::initializer_list<std::pair<std::string_view, std::string_view>>);
+
+private:
+  friend struct Metric;
+  std::string_view name;
+  std::string_view help;
+};
+
+// All std::string_view's should point to static memory
+Family<Counter> create_counter(std::string_view name, std::string_view help);
+Family<Gauge> create_gauge(std::string_view name, std::string_view help);
+Family<Histogram> create_histogram(std::string_view name, std::string_view help,
+                                   std::initializer_list<double> buckets);
+
+std::span<std::string_view> render(ArenaAllocator &);
+
+} // namespace metric