add sys.uptime metric (#134)

This metric tracks the number of seconds that have elapsed since the last system
boot.

bin/atlas-agent.cc

@@ -66,6 +66,7 @@ static void gather_slow_titus_metrics(CGroup* cGroup, Proc* proc, Disk* disk, Aw
   proc->network_stats();
   proc->process_stats();
   proc->snmp_stats();
+  proc->uptime_stats();
 }
 #else
 static void gather_peak_system_metrics(Proc* proc) { proc->peak_cpu_stats(); }
@@ -89,6 +90,7 @@ static void gather_slow_system_metrics(Proc* proc, Disk* disk, Ethtool* ethtool,
   proc->process_stats();
   proc->snmp_stats();
   proc->socket_stats();
+  proc->uptime_stats();
   proc->vmstats();
 }
 #endif

lib/internal/proc.inc

@@ -554,6 +554,15 @@ inline void set_if_present(const std::unordered_map<std::string, int64_t>& stats
   }
 }
 
+template <typename Reg>
+void Proc<Reg>::uptime_stats() noexcept {
+  static auto sys_uptime = registry_->GetGauge("sys.uptime");
+  // uptime values are in seconds, reported as doubles, but given how large they will be over
+  // time, the 10ths of a second will not matter for the purpose of producing this metric
+  auto uptime_seconds = read_num_vector_from_file(path_prefix_, "uptime");
+  sys_uptime->Set(uptime_seconds[0]);
+}
+
 template <typename Reg>
 void Proc<Reg>::vmstats() noexcept {
   static auto processes = registry_->GetMonotonicCounter("vmstat.procs.count");

lib/proc.h

@@ -18,6 +18,7 @@ class Proc {
   void memory_stats() noexcept;
   void process_stats() noexcept;
   void socket_stats() noexcept;
+  void uptime_stats() noexcept;
   void vmstats() noexcept;
   [[nodiscard]] bool is_container() const noexcept;
 

lib/proc_test.cc

@@ -172,6 +172,17 @@ TEST(Proc, CpuStats) {
   EXPECT_EQ(17, ms2.size());
 }
 
+TEST(Proc, UptimeStats) {
+  Registry registry;
+  spectator::Tags extra{{"nf.test", "extra"}};
+  Proc proc{&registry, extra, "testdata/resources/proc"};
+  proc.uptime_stats();
+  auto ms = my_measurements(&registry);
+  auto ms_map = measurements_to_map(ms, "proto");
+  expect_value(&ms_map, "sys.uptime|gauge", 517407);
+  EXPECT_TRUE(ms_map.empty());
+}
+
 TEST(Proc, VmStats) {
   Registry registry;
   spectator::Tags extra{{"nf.test", "extra"}};

testdata/resources/proc/uptime

@@ -0,0 +1 @@
+517407.10 4128064.05