Add OS-reported metrics for cached/buffered memory.

service/src/main/java/org/whispersystems/textsecuregcm/metrics/OperatingSystemMemoryGauge.java

@@ -0,0 +1,43 @@
+package org.whispersystems.textsecuregcm.metrics;
+
+import com.codahale.metrics.Gauge;
+import com.google.common.annotations.VisibleForTesting;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.FileReader;
+import java.io.IOException;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.stream.Stream;
+
+public class OperatingSystemMemoryGauge implements Gauge<Long> {
+
+    private final String metricName;
+
+    private static final File MEMINFO_FILE = new File("/proc/meminfo");
+    private static final Pattern MEMORY_METRIC_PATTERN = Pattern.compile("^([^:]+):\\s+([0-9]+).*$");
+
+    public OperatingSystemMemoryGauge(final String metricName) {
+        this.metricName = metricName;
+    }
+
+    @Override
+    public Long getValue() {
+        try (final BufferedReader bufferedReader = new BufferedReader(new FileReader(MEMINFO_FILE))) {
+            return getValue(bufferedReader.lines());
+        } catch (final IOException e) {
+            return 0L;
+        }
+    }
+
+    @VisibleForTesting
+    long getValue(final Stream<String> lines) {
+        return lines.map(MEMORY_METRIC_PATTERN::matcher)
+                    .filter(Matcher::matches)
+                    .filter(matcher -> this.metricName.equalsIgnoreCase(matcher.group(1)))
+                    .map(matcher -> Long.parseLong(matcher.group(2), 10))
+                    .findFirst()
+                    .orElse(0L);
+    }
+}