hydra-queue-runner: add simple "up" exporter

There are probably better ways to achieve this (and will likely need to
be refactored a bit to support further metrics).

flake.nix

@@ -565,6 +565,7 @@
               (if lib.versionAtLeast lib.version "20.03pre"
               then nlohmann_json
               else nlohmann_json.override { multipleHeaders = true; })
+              prometheus-cpp
             ];
 
           checkInputs = [

src/hydra-queue-runner/Makefile.am

@@ -4,5 +4,5 @@ hydra_queue_runner_SOURCES = hydra-queue-runner.cc queue-monitor.cc dispatcher.c
  builder.cc build-result.cc build-remote.cc \
  build-result.hh counter.hh state.hh db.hh \
  nar-extractor.cc nar-extractor.hh
-hydra_queue_runner_LDADD = $(NIX_LIBS) -lpqxx
+hydra_queue_runner_LDADD = $(NIX_LIBS) -lpqxx -lprometheus-cpp-pull -lprometheus-cpp-core
 hydra_queue_runner_CXXFLAGS = $(NIX_CFLAGS) -Wall -I ../libhydra -Wno-deprecated-declarations

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -6,6 +6,10 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+#include <prometheus/exposer.h>
+#include <prometheus/registry.h>
+#include <prometheus/gauge.h>
+
 #include "state.hh"
 #include "build-result.hh"
 #include "store-api.hh"
@@ -854,6 +858,25 @@ int main(int argc, char * * argv)
     return handleExceptions(argv[0], [&]() {
         initNix();
 
+        /* Export a simple "up" metric, to allow monitoring that we're
+           still alive. */
+        std::thread([&]() {
+            prometheus::Exposer exposer{"127.0.0.1:8080"};
+
+            // @note it's the users responsibility to keep the object alive
+            auto registry = std::make_shared<prometheus::Registry>();
+
+            auto& running = prometheus::BuildGauge()
+                .Name("hydra_queue_runner_running")
+                .Help("Whether the queue runner is currently running")
+                .Register(*registry);
+
+            exposer.RegisterCollectable(registry);
+            running.Add({}).Set(1);
+
+            while (true) { }
+        }).detach();
+
         signal(SIGINT, SIG_DFL);
         signal(SIGTERM, SIG_DFL);
         signal(SIGHUP, SIG_DFL);