feat: Add jvm.file_descriptor.count metric to JMX instrumentation (#13722)

Co-authored-by: SylvainJuge <[email protected]>

Author: schmikei

instrumentation/jmx-metrics/library/jvm.md

@@ -5,7 +5,7 @@ Here is the list of metrics based on MBeans exposed by the JVM and that are defi
 Those metrics are defined in the [JVM runtime metrics semantic conventions](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/).
 
 | Metric Name                                                                                                                           | semconv maturity | Type          | Attributes                            | Description                                        |
-|---------------------------------------------------------------------------------------------------------------------------------------|:-----------------|---------------|---------------------------------------|----------------------------------------------------|
+| ------------------------------------------------------------------------------------------------------------------------------------- | :--------------- | ------------- | ------------------------------------- | -------------------------------------------------- |
 | [jvm.memory.used](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmmemoryused)                              | stable           | UpDownCounter | jvm.memory.pool.name, jvm.memory.type | Used memory                                        |
 | [jvm.memory.committed](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmmemorycommitted)                    | stable           | UpDownCounter | jvm.memory.pool.name, jvm.memory.type | Committed memory                                   |
 | [jvm.memory.limit](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmmemorylimit)                            | stable           | UpDownCounter | jvm.memory.pool.name, jvm.memory.type | Max obtainable memory                              |
@@ -17,6 +17,7 @@ Those metrics are defined in the [JVM runtime metrics semantic conventions](http
 | [jvm.class.count](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmclasscount)                              | stable           | UpDownCounter |                                       | Classes currently loaded count                     |
 | [jvm.cpu.count](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmcpucount)                                  | stable           | UpDownCounter |                                       | Number of CPUs available                           |
 | [jvm.cpu.recent_utilization](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmcpurecent_utilization)        | stable           | Gauge         |                                       | Recent CPU utilization for process reported by JVM |
+| [jvm.file_descriptor.count](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmfile_descriptorcount)          | experimental     | UpDownCounter |                                       | Number of open file descriptors                    |
 | [jvm.system.cpu.load_1m](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmsystemcpuload_1m)                 | experimental     | Gauge         |                                       | Average CPU load reported by JVM                   |
 | [jvm.system.cpu.recent_utilization](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmcpurecent_utilization) | experimental     | Gauge         |                                       | Recent CPU utilization reported by JVM             |
 | [jvm.buffer.memory.used](https://opentelemetry.io/docs/specs/semconv/runtime/jvm-metrics/#metric-jvmbuffermemoryused)                 | experimental     | UpDownCounter | jvm.buffer.pool.name                  | Memory used by buffers                             |

instrumentation/jmx-metrics/library/src/main/resources/jmx/rules/jvm.yaml

@@ -96,6 +96,12 @@ rules:
         type: gauge
         unit: '1'
         desc: Recent CPU utilization for the whole system as reported by the JVM.
+      # jvm.file_descriptor.count (experimental)
+      OpenFileDescriptorCount:
+        metric: file_descriptor.count
+        type: updowncounter
+        unit: "{file_descriptor}"
+        desc: Number of open file descriptors as reported by the JVM.
 
   - bean: java.nio:name=*,type=BufferPool
     prefix: jvm.buffer.

instrumentation/jmx-metrics/library/src/test/java/io/opentelemetry/instrumentation/jmx/rules/JvmTargetSystemTest.java

@@ -162,6 +162,14 @@ void testJvmMetrics(String image) {
                         .hasUnit("1")
                         .isGauge()
                         .hasDataPointsWithoutAttributes())
+            .add(
+                "jvm.file_descriptor.count",
+                metric ->
+                    metric
+                        .hasDescription("Number of open file descriptors as reported by the JVM.")
+                        .hasUnit("{file_descriptor}")
+                        .isUpDownCounter()
+                        .hasDataPointsWithoutAttributes())
             .add(
                 "jvm.system.cpu.load_1m",
                 metric ->