add header-only prometheus lib to ext

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

@@ -0,0 +1,67 @@
+
+#include <prometheus/registry.h>
+#include <prometheus/counter.h>
+#include <prometheus/text_serializer.h>
+
+#include <array>
+#include <chrono>
+#include <cstdlib>
+#include <memory>
+#include <string>
+#include <thread>
+#include <iostream>
+
+int main() {
+
+  using namespace prometheus;
+
+  // create a metrics registry
+  // @note it's the users responsibility to keep the object alive
+  auto registry = std::make_shared<Registry>();
+
+  // 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/
+  auto& packet_counter = BuildCounter()
+                             .Name("observed_packets_total")
+                             .Help("Number of observed packets")
+                             .Register(*registry);
+
+  // add and remember dimensional data, incrementing those is very cheap
+  auto& tcp_rx_counter = packet_counter.Add({ {"protocol", "tcp"}, {"direction", "rx"} });
+  auto& tcp_tx_counter = packet_counter.Add({ {"protocol", "tcp"}, {"direction", "tx"} });
+  auto& udp_rx_counter = packet_counter.Add({ {"protocol", "udp"}, {"direction", "rx"} });
+  auto& udp_tx_counter = packet_counter.Add({ {"protocol", "udp"}, {"direction", "tx"} });
+
+  // add a counter whose dimensional data is not known at compile time
+  // nevertheless dimensional values should only occur in low cardinality:
+  // https://prometheus.io/docs/practices/naming/#labels
+  auto& http_requests_counter = BuildCounter()
+                                    .Name("http_requests_total")
+                                    .Help("Number of HTTP requests")
+                                    .Register(*registry);
+
+  // ask the exposer to scrape the registry on incoming HTTP requests
+  //exposer.RegisterCollectable(registry);
+
+  for ( ;; ) {
+    std::this_thread::sleep_for(std::chrono::seconds(1));
+    const auto random_value = std::rand();
+
+    if (random_value & 1) tcp_rx_counter.Increment();
+    if (random_value & 2) tcp_tx_counter.Increment();
+    if (random_value & 4) udp_rx_counter.Increment(10);
+    if (random_value & 8) udp_tx_counter.Increment(10);
+
+    const std::array<std::string, 4> methods = { "GET", "PUT", "POST", "HEAD" };
+    auto method = methods.at(static_cast<std::size_t>(random_value) % methods.size());
+    // dynamically calling Family<T>.Add() works but is slow and should be avoided
+    http_requests_counter.Add({ {"method", method} }).Increment();
+
+    TextSerializer text_serializer;
+    text_serializer.Serialize(std::cout, registry->Collect());
+
+  }
+}