add experimental metric to jvm runtime metrics

Author: schmikei

instrumentation/runtime-telemetry/runtime-telemetry-java8/library/src/main/java/io/opentelemetry/instrumentation/runtimemetrics/java8/internal/ExperimentalFileDescriptor.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.runtimemetrics.java8.internal;
+
+import io.opentelemetry.api.OpenTelemetry;
+import io.opentelemetry.api.metrics.Meter;
+import io.opentelemetry.instrumentation.runtimemetrics.java8.internal.JmxRuntimeMetricsUtil;
+import java.lang.management.ManagementFactory;
+import java.lang.management.OperatingSystemMXBean;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * This class is internal and is hence not for public use. Its APIs are unstable and can change at
+ * any time.
+ */
+public final class ExperimentalFileDescriptor {
+
+  /** Register observers for java runtime file descriptor metrics. */
+  public static List<AutoCloseable> registerObservers(OpenTelemetry openTelemetry) {
+    return registerObservers(openTelemetry, ManagementFactory.getOperatingSystemMXBean());
+  }
+
+  // Visible for testing
+  static List<AutoCloseable> registerObservers(
+      OpenTelemetry openTelemetry, OperatingSystemMXBean osBean) {
+    Meter meter = JmxRuntimeMetricsUtil.getMeter(openTelemetry);
+    List<AutoCloseable> observables = new ArrayList<>();
+
+    if (osBean instanceof com.sun.management.UnixOperatingSystemMXBean) {
+      observables.add(
+          meter
+              .upDownCounterBuilder("jvm.file_descriptor.count")
+              .setDescription("Number of open file descriptors as reported by the JVM.")
+              .setUnit("{file_descriptor}")
+              .buildWithCallback(
+                  observableMeasurement -> {
+                    long openFileDescriptorCount =
+                        ((com.sun.management.UnixOperatingSystemMXBean) osBean)
+                            .getOpenFileDescriptorCount();
+                    if (openFileDescriptorCount >= 0) {
+                      observableMeasurement.record(openFileDescriptorCount);
+                    }
+                  }));
+    }
+
+    return observables;
+  }
+
+  private ExperimentalFileDescriptor() {}
+} 

instrumentation/runtime-telemetry/runtime-telemetry-java8/library/src/main/java/io/opentelemetry/instrumentation/runtimemetrics/java8/internal/JmxRuntimeMetricsFactory.java

@@ -33,6 +33,7 @@ public static List<AutoCloseable> buildObservables(
       observables.addAll(ExperimentalBufferPools.registerObservers(openTelemetry));
       observables.addAll(ExperimentalCpu.registerObservers(openTelemetry));
       observables.addAll(ExperimentalMemoryPools.registerObservers(openTelemetry));
+      observables.addAll(ExperimentalFileDescriptor.registerObservers(openTelemetry));
     }
     return observables;
   }

instrumentation/runtime-telemetry/runtime-telemetry-java8/library/src/test/java/io/opentelemetry/instrumentation/runtimemetrics/java8/internal/ExperimentalFileDescriptorTest.java

@@ -0,0 +1,84 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.runtimemetrics.java8.internal;
+
+import static io.opentelemetry.instrumentation.runtimemetrics.java8.ScopeUtil.EXPECTED_SCOPE;
+import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
+import static org.mockito.Mockito.when;
+
+import com.sun.management.UnixOperatingSystemMXBean;
+import io.opentelemetry.instrumentation.testing.junit.InstrumentationExtension;
+import io.opentelemetry.instrumentation.testing.junit.LibraryInstrumentationExtension;
+import java.lang.management.ManagementFactory;
+import java.lang.management.OperatingSystemMXBean;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.extension.RegisterExtension;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+@ExtendWith(MockitoExtension.class)
+class ExperimentalFileDescriptorTest {
+
+  @RegisterExtension
+  static final InstrumentationExtension testing = LibraryInstrumentationExtension.create();
+
+  @Mock private UnixOperatingSystemMXBean osBean;
+
+  @BeforeEach
+  void setUp() {
+    // Skip tests on non-Unix systems since UnixOperatingSystemMXBean is only available on Unix
+    OperatingSystemMXBean realOsBean = ManagementFactory.getOperatingSystemMXBean();
+    assumeTrue(realOsBean instanceof UnixOperatingSystemMXBean, 
+        "Skipping test: UnixOperatingSystemMXBean is only available on Unix systems");
+  }
+
+  @Test
+  // verify that mock is called with the correct value
+  void registerObservers() {
+    when(osBean.getOpenFileDescriptorCount()).thenReturn(42L);
+    ExperimentalFileDescriptor.registerObservers(testing.getOpenTelemetry(), osBean);
+
+    testing.waitAndAssertMetrics(
+        "io.opentelemetry.runtime-telemetry-java8",
+        "jvm.file_descriptor.count",
+        metrics ->
+            metrics.anySatisfy(
+                metricData ->
+                    assertThat(metricData)
+                        .hasInstrumentationScope(EXPECTED_SCOPE)
+                        .hasDescription("Number of open file descriptors as reported by the JVM.")
+                        .hasUnit("{file_descriptor}")
+                        .hasLongSumSatisfying(
+                            sum ->
+                                sum.hasPointsSatisfying(
+                                    point -> point.hasValue(42)))));
+  }
+
+  @Test
+  // Verify that no metrics are emitted with non-zero values
+  void registerObservers_NegativeValue() {
+    when(osBean.getOpenFileDescriptorCount()).thenReturn(-1L);
+    ExperimentalFileDescriptor.registerObservers(testing.getOpenTelemetry(), osBean);
+
+    testing.waitAndAssertMetrics(
+        "io.opentelemetry.runtime-telemetry-java8",
+        "jvm.file_descriptor.count",
+        metrics ->
+            metrics.allSatisfy(
+                metricData ->
+                    assertThat(metricData)
+                        .hasInstrumentationScope(EXPECTED_SCOPE)
+                        .hasDescription("Number of open file descriptors as reported by the JVM.")
+                        .hasUnit("{file_descriptor}")
+                        .hasLongSumSatisfying(
+                            sum ->
+                                sum.hasPointsSatisfying(
+                                    point -> point.hasValue(0)))));
+  }
+}