feat: Add `jvm.file_descriptor.count` metric to jvm experimental metrics (#2139)

.chloggen/add-file-descriptor-metric-to-jvm.yaml

@@ -0,0 +1,22 @@
+# Use this changelog template to create an entry for release notes.
+#
+# If your change doesn't affect end users you should instead start
+# your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the area of concern in the attributes-registry, (e.g. http, cloud, db)
+component: jvm
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add `jvm.file_descriptor.count` as an in-development metric to track the number of open file descriptors as reported by the JVM.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+# The values here must be integers.
+issues: [1838]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

docs/runtime/jvm-metrics.md

@@ -34,6 +34,7 @@ This document describes semantic conventions for JVM metrics in OpenTelemetry.
   - [Metric: `jvm.buffer.memory.used`](#metric-jvmbuffermemoryused)
   - [Metric: `jvm.buffer.memory.limit`](#metric-jvmbuffermemorylimit)
   - [Metric: `jvm.buffer.count`](#metric-jvmbuffercount)
+  - [Metric: `jvm.file_descriptor.count`](#metric-jvmfile_descriptorcount)
 
 <!-- tocstop -->
 
@@ -600,6 +601,27 @@ This metric is obtained from [`BufferPoolMXBean#getCount()`](https://docs.oracle
 <!-- END AUTOGENERATED TEXT -->
 <!-- endsemconv -->
 
+### Metric: `jvm.file_descriptor.count`
+
+This metric is [Opt-In][MetricOptIn].
+This metric is obtained from [`UnixOperatingSystemMXBean#getOpenFileDescriptorCount()`](https://docs.oracle.com/en/java/javase/17/docs/api/jdk.management/com/sun/management/UnixOperatingSystemMXBean.html#getOpenFileDescriptorCount()).
+
+<!-- semconv metric.jvm.file_descriptor.count -->
+<!-- NOTE: THIS TEXT IS AUTOGENERATED. DO NOT EDIT BY HAND. -->
+<!-- see templates/registry/markdown/snippet.md.j2 -->
+<!-- prettier-ignore-start -->
+<!-- markdownlint-capture -->
+<!-- markdownlint-disable -->
+
+| Name     | Instrument Type | Unit (UCUM) | Description    | Stability |
+| -------- | --------------- | ----------- | -------------- | --------- |
+| `jvm.file_descriptor.count` | UpDownCounter | `{file_descriptor}` | Number of open file descriptors as reported by the JVM. | ![Development](https://img.shields.io/badge/-development-blue) |
+
+<!-- markdownlint-restore -->
+<!-- prettier-ignore-end -->
+<!-- END AUTOGENERATED TEXT -->
+<!-- endsemconv -->
+
 [DocumentStatus]: https://opentelemetry.io/docs/specs/otel/document-status
 [MetricOptIn]: /docs/general/metric-requirement-level.md#opt-in
 [MetricRecommended]: /docs/general/metric-requirement-level.md#recommended

model/jvm/metrics-experimental.yaml

@@ -67,3 +67,11 @@ groups:
     brief: "Number of buffers in the pool."
     instrument: updowncounter
     unit: "{buffer}"
+
+  - id: metric.jvm.file_descriptor.count
+    type: metric
+    metric_name: jvm.file_descriptor.count
+    stability: development
+    brief: "Number of open file descriptors as reported by the JVM."
+    instrument: updowncounter
+    unit: "{file_descriptor}"