add header-only prometheus lib to ext

ext/prometheus-cpp-lite-1.0/examples/save_to_file_example.cpp

@@ -0,0 +1,43 @@
+#include <prometheus/registry.h>
+#include <prometheus/counter.h>
+#include <prometheus/save_to_file.h>
+
+#include <array>
+#include <chrono>
+#include <cstdlib>
+#include <memory>
+#include <string>
+#include <thread>
+#include <iostream>
+
+int main() {
+
+  using namespace prometheus;
+
+  // for clarity, we deduce the required types
+  using Metric = Counter<uint64_t>;
+
+  using Family = Metric::Family;
+
+  // create a metrics registry
+  // @note it's the users responsibility to keep the object alive
+  std::shared_ptr<Registry> registry_ptr = std::make_shared<Registry>();
+
+  SaveToFile saver( registry_ptr, std::chrono::seconds(5), "./metrics.prom" );
+
+  // add a new counter family to the registry (families combine values with the
+  // same name, but distinct label dimensions)
+  //
+  // @note please follow the metric-naming best-practices:
+  // https://prometheus.io/docs/practices/naming/
+  Family& family { Family::Build(*registry_ptr, "our_metric", "some metric") };
+
+  // add and remember dimensional data, incrementing those is very cheap
+  Metric& metric { family.Add({}) };
+
+  for (;; ) {
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    const int random_value = std::rand();
+    metric += random_value % 10;
+  }
+}