Proc metrics: add average system load for the last minute

Change-Id: I12d74ea28f7f8f52c16d53236ecca6582df351b7

Documentation/metrics.txt

@@ -35,6 +35,7 @@ split up by update type (create+replace, and autoclose)
 * `proc/uptime`: Uptime of the Gerrit process.
 * `proc/cpu/num_cores`: Number of processors available to the Java virtual machine.
 * `proc/cpu/usage`: CPU time used by the Gerrit process.
+* `proc/cpu/system_load`: System load average for the last minute.
 * `proc/num_open_fds`: Number of open file descriptors.
 * `proc/jvm/memory/heap_committed`: Amount of memory guaranteed for user objects.
 * `proc/jvm/memory/heap_used`: Amount of memory holding user objects.

java/com/google/gerrit/metrics/proc/ProcMetricModule.java

@@ -29,6 +29,7 @@ import java.lang.management.GarbageCollectorMXBean;
 import java.lang.management.ManagementFactory;
 import java.lang.management.MemoryMXBean;
 import java.lang.management.MemoryUsage;
+import java.lang.management.OperatingSystemMXBean;
 import java.lang.management.ThreadMXBean;
 import java.util.concurrent.TimeUnit;
 
@@ -38,6 +39,7 @@ public class ProcMetricModule extends MetricModule {
     buildLabel(metrics);
     procUptime(metrics);
     procCpuUsage(metrics);
+    procCpuLoad(metrics);
     procJvmGc(metrics);
     procJvmMemory(metrics);
     procJvmThread(metrics);
@@ -97,6 +99,18 @@ public class ProcMetricModule extends MetricModule {
         Runtime.getRuntime()::availableProcessors);
   }
 
+  private void procCpuLoad(MetricMaker metrics) {
+    OperatingSystemMXBean provider =
+        ManagementFactory.getPlatformMXBean(OperatingSystemMXBean.class);
+    if (provider.getSystemLoadAverage() != -1) {
+      metrics.newCallbackMetric(
+          "proc/cpu/system_load",
+          Double.class,
+          new Description("System load average for the last minute").setGauge(),
+          provider::getSystemLoadAverage);
+    }
+  }
+
   private void procJvmMemory(MetricMaker metrics) {
     CallbackMetric0<Long> heapCommitted =
         metrics.newCallbackMetric(