Add stat metrics collector (#597)

* Add stat metrics collector

* Fix example-plugin test

example-plugin/build.gradle

@@ -34,6 +34,7 @@ dependencies {
     testImplementation("com.yelp.nrtsearch:server:${nrtsearchVersion}:tests")
     testImplementation "junit:junit:4.13"
     testImplementation "org.assertj:assertj-core:3.19.0"
+    testImplementation "io.grpc:grpc-testing:1.46.0"
 }
 
 distributions {

src/main/java/com/yelp/nrtsearch/server/grpc/LuceneServer.java

@@ -233,6 +233,7 @@ private void registerMetrics(GlobalState globalState) {
     DeadlineMetrics.register(collectorRegistry);
     // register directory size metrics
     new DirSizeCollector(globalState).register(collectorRegistry);
+    new ProcStatCollector().register(collectorRegistry);
   }
 
   /** Main launches the server from the command line. */

src/main/java/com/yelp/nrtsearch/server/monitoring/ProcStatCollector.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2023 Yelp Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.yelp.nrtsearch.server.monitoring;
+
+import io.prometheus.client.Collector;
+import io.prometheus.client.GaugeMetricFamily;
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/** Collect stat metrics for this process from procfs. */
+public class ProcStatCollector extends Collector {
+  private static final Logger logger = LoggerFactory.getLogger(ProcStatCollector.class);
+  private static final String STAT_PATH_FORMAT = "/proc/%d/stat";
+  private static final int MINOR_FAULT_INDEX = 9;
+  private static final int CHILD_MINOR_FAULT_INDEX = 10;
+  private static final int MAJOR_FAULT_INDEX = 11;
+  private static final int CHILD_MAJOR_FAULT_INDEX = 12;
+
+  private final File statFile;
+  private boolean collectStat;
+
+  public ProcStatCollector() {
+    long pid = ProcessHandle.current().pid();
+    statFile = new File(String.format(STAT_PATH_FORMAT, pid));
+    collectStat = statFile.exists();
+  }
+
+  @Override
+  public List<MetricFamilySamples> collect() {
+    if (collectStat) {
+      List<MetricFamilySamples> mfs = new ArrayList<>();
+
+      List<String> lines;
+      try {
+        lines = Files.readAllLines(statFile.toPath());
+      } catch (IOException e) {
+        logger.warn("Error reading stat info", e);
+        collectStat = false;
+        return Collections.emptyList();
+      }
+
+      try {
+        for (String line : lines) {
+          String[] components = line.split("\\s+");
+          if (components.length > CHILD_MAJOR_FAULT_INDEX) {
+            mfs.add(
+                new GaugeMetricFamily(
+                    "nrt_stat_minor_faults",
+                    "Total number of minor page faults",
+                    Double.parseDouble(components[MINOR_FAULT_INDEX])));
+            mfs.add(
+                new GaugeMetricFamily(
+                    "nrt_stat_child_minor_faults",
+                    "Total number of minor page faults by waited-for children",
+                    Double.parseDouble(components[CHILD_MINOR_FAULT_INDEX])));
+            mfs.add(
+                new GaugeMetricFamily(
+                    "nrt_stat_major_faults",
+                    "Total number of major page faults",
+                    Double.parseDouble(components[MAJOR_FAULT_INDEX])));
+            mfs.add(
+                new GaugeMetricFamily(
+                    "nrt_stat_child_major_faults",
+                    "Total number of major page faults by waited-for children",
+                    Double.parseDouble(components[CHILD_MAJOR_FAULT_INDEX])));
+          }
+        }
+      } catch (Exception e) {
+        logger.warn("Error parsing stat result", e);
+        collectStat = false;
+        return Collections.emptyList();
+      }
+
+      return mfs;
+    } else {
+      return Collections.emptyList();
+    }
+  }
+}