otel: add new module

Adds support for OpenTelemetry tracing, as described in the included
docs.

Custom instrumentation is not yet supported.

Author: bengl

doc/api/cli.md

@@ -3531,6 +3531,47 @@ added: v6.11.0
 
 When set to `1`, process warnings are silenced.
 
+### `NODE_OTEL=value`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+When set to a non-empty value, enables the built-in OpenTelemetry tracing
+subsystem using the default collector endpoint (`http://localhost:4318`). Also
+makes the `node:otel` module available without the `--experimental-otel` flag.
+If `NODE_OTEL_ENDPOINT` is also set, it takes precedence for the endpoint. See
+the [`node:otel`][] documentation for details.
+
+### `NODE_OTEL_ENDPOINT=url`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+When set to a non-empty value, enables the built-in OpenTelemetry tracing
+subsystem and directs spans to the specified OTLP/HTTP collector endpoint. The
+`/v1/traces` path is appended automatically. Also makes the `node:otel` module
+available without the `--experimental-otel` flag. See the [`node:otel`][]
+documentation for details.
+
+### `NODE_OTEL_FILTER=module[,…]`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+Comma-separated list of core modules to instrument when OpenTelemetry tracing
+is active. When not set, all supported modules are instrumented. Supported
+values: `node:http`, `node:undici`, `node:fetch`. See the [`node:otel`][]
+documentation for details.
+
 ### `NODE_OPTIONS=options...`
 
 <!-- YAML
@@ -4240,6 +4281,7 @@ node --stack-trace-limit=12 -p -e "Error.stackTraceLimit" # prints 12
 [`import.meta.url`]: esm.md#importmetaurl
 [`import` specifier]: esm.md#import-specifiers
 [`net.getDefaultAutoSelectFamilyAttemptTimeout()`]: net.md#netgetdefaultautoselectfamilyattempttimeout
+[`node:otel`]: otel.md
 [`node:sqlite`]: sqlite.md
 [`process.setUncaughtExceptionCaptureCallback()`]: process.md#processsetuncaughtexceptioncapturecallbackfn
 [`tls.DEFAULT_MAX_VERSION`]: tls.md#tlsdefault_max_version

doc/api/otel.md

@@ -0,0 +1,235 @@
+# OpenTelemetry
+
+<!--introduced_in=REPLACEME-->
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+<!-- source_link=lib/otel.js -->
+
+The `node:otel` module provides built-in [OpenTelemetry][] tracing support for
+Node.js core components. When enabled, it automatically creates spans for HTTP
+server and client operations and exports them using the [OTLP/HTTP JSON][]
+protocol.
+
+To access it:
+
+```cjs
+const otel = require('node:otel');
+```
+
+```mjs
+import otel from 'node:otel';
+```
+
+This module is only available under the `node:` scheme. It requires the
+`--experimental-otel` CLI flag or one of the `NODE_OTEL` / `NODE_OTEL_ENDPOINT`
+environment variables.
+
+## Activation
+
+There are two ways to activate OpenTelemetry tracing:
+
+### Environment variables
+
+Setting `NODE_OTEL=1` enables tracing with the default collector endpoint
+(`http://localhost:4318`):
+
+```bash
+NODE_OTEL=1 node app.js
+```
+
+To use a custom collector endpoint, set `NODE_OTEL_ENDPOINT` instead (or in
+addition):
+
+```bash
+NODE_OTEL_ENDPOINT=http://collector.example.com:4318 node app.js
+```
+
+Both variables also make the `node:otel` module available without requiring the
+`--experimental-otel` flag.
+
+### Programmatic API
+
+```cjs
+const otel = require('node:otel');
+
+// Start with the default endpoint (http://localhost:4318):
+otel.start();
+
+// Or with a custom endpoint:
+otel.start({ endpoint: 'http://collector.example.com:4318' });
+
+// ... application code ...
+otel.stop();
+```
+
+## Environment variables
+
+### `NODE_OTEL`
+
+When set to a non-empty value, enables the OpenTelemetry tracing subsystem
+using the default collector endpoint (`http://localhost:4318`). If
+`NODE_OTEL_ENDPOINT` is also set, it takes precedence.
+
+### `NODE_OTEL_ENDPOINT`
+
+When set to a non-empty value, enables the OpenTelemetry tracing subsystem and
+directs spans to the specified OTLP collector endpoint. The endpoint should be
+the base URL of an
+OTLP/HTTP collector (e.g. `http://localhost:4318`). The `/v1/traces` path is
+appended automatically.
+
+### `NODE_OTEL_FILTER`
+
+Accepts a comma-separated list of core modules to instrument. When not set, all
+supported modules are instrumented. For example, setting
+`NODE_OTEL_FILTER=node:http` would enable tracing only for the `node:http`
+module.
+
+Supported module filter values:
+
+* `node:http` — HTTP server and client operations
+* `node:undici` — Undici HTTP client operations
+* `node:fetch` — Fetch API operations (alias for undici)
+
+### `OTEL_SERVICE_NAME`
+
+Standard OpenTelemetry environment variable used to set the service name in
+exported resource attributes. Defaults to `node-<pid>`.
+
+## `otel.start(options)`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `endpoint` {string} The OTLP collector endpoint URL.
+    **Default:** `'http://localhost:4318'`.
+  * `filter` {string|string\[]} Optional comma-separated string or array of
+    core module names to instrument (e.g. `['node:http']` or
+    `'node:http,node:undici'`). When omitted, all supported modules are
+    instrumented.
+  * `maxBufferSize` {number} Maximum number of spans buffered in memory before
+    triggering an immediate flush to the collector. Must be a positive integer.
+    **Default:** `100`.
+  * `flushInterval` {number} Interval in milliseconds between periodic flushes
+    of buffered spans to the collector. Must be a positive integer.
+    **Default:** `10000`.
+
+Enables the OpenTelemetry tracing subsystem. Spans are created for supported
+core module operations and exported to the specified collector using the
+OTLP/HTTP JSON protocol.
+
+If tracing is already active, calling `start()` will stop the current session
+and start a new one with the provided options.
+
+## `otel.stop()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Disables the OpenTelemetry tracing subsystem. Any buffered spans are sent to
+the collector before stopping. Because the export is asynchronous, delivery is
+not confirmed before `stop()` returns. After calling `stop()`, no new spans are
+created.
+
+Calling `stop()` when tracing is not active is a no-op.
+
+## `otel.active`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {boolean}
+
+Returns `true` if the OpenTelemetry tracing subsystem is currently active.
+
+## Instrumented operations
+
+When tracing is active, spans are automatically created for the following
+operations:
+
+### HTTP server
+
+A span with kind `SERVER` is created for each incoming HTTP request. The span
+starts when the request is received and ends when the response finishes. If the
+client disconnects before the response completes, the span ends with an error
+status.
+
+Server spans receive error status (`STATUS_ERROR`) for 5xx response codes. 4xx
+responses are not treated as server errors per OpenTelemetry semantic
+conventions.
+
+Attributes set on server spans:
+
+| Attribute                   | Description                      | Condition                     |
+| --------------------------- | -------------------------------- | ----------------------------- |
+| `http.request.method`       | HTTP method (e.g. `GET`, `POST`) | Always                        |
+| `url.path`                  | Request URL path (without query) | Always                        |
+| `url.query`                 | Query string (without `?`)       | When query string is present  |
+| `url.scheme`                | `http` or `https`                | Always                        |
+| `server.address`            | Host header value                | When `Host` header is present |
+| `network.protocol.version`  | HTTP version (e.g. `1.1`)        | Always                        |
+| `http.response.status_code` | Response status code             | When response finishes        |
+| `error.type`                | HTTP status code as string       | On 5xx responses              |
+
+### HTTP client
+
+A span with kind `CLIENT` is created for each outgoing HTTP request made via
+`node:http`. The span starts when the request is created and ends when the
+response finishes or an error occurs.
+
+Client spans receive error status (`STATUS_ERROR`) for 4xx and 5xx response
+codes. On connection errors, an `exception` event is added to the span with
+`exception.type`, `exception.message`, and `exception.stacktrace` attributes.
+
+Attributes set on client spans:
+
+| Attribute                   | Description               | Condition                 |
+| --------------------------- | ------------------------- | ------------------------- |
+| `http.request.method`       | HTTP method               | Always                    |
+| `url.full`                  | Full request URL          | Always                    |
+| `server.address`            | Target host               | Always                    |
+| `server.port`               | Target port               | When available            |
+| `http.response.status_code` | Response status code      | When response is received |
+| `network.protocol.version`  | HTTP version              | When response is received |
+| `error.type`                | Status code or error name | On 4xx/5xx or errors      |
+
+### Undici/Fetch client
+
+A span with kind `CLIENT` is created for each outgoing request made via
+`fetch()` or undici's `request()`.
+
+Error status and `exception` event behavior is the same as for HTTP client
+spans above.
+
+Attributes set on undici/fetch client spans:
+
+| Attribute                   | Description               | Condition                 |
+| --------------------------- | ------------------------- | ------------------------- |
+| `http.request.method`       | HTTP method               | Always                    |
+| `url.full`                  | Full request URL          | Always                    |
+| `server.address`            | Target origin             | Always                    |
+| `http.response.status_code` | Response status code      | When response is received |
+| `error.type`                | Status code or error name | On 4xx/5xx or errors      |
+
+## W3C Trace Context propagation
+
+The tracing subsystem automatically propagates [W3C Trace Context][] across HTTP
+boundaries:
+
+* **Incoming requests**: The `traceparent` header is read from incoming HTTP
+  requests. Child spans created during request processing inherit the trace ID.
+* **Outgoing requests**: The `traceparent` header is injected into outgoing HTTP
+  and undici/fetch requests, enabling distributed tracing across services.
+
+[OTLP/HTTP JSON]: https://opentelemetry.io/docs/specs/otlp/#otlphttp
+[OpenTelemetry]: https://opentelemetry.io/
+[W3C Trace Context]: https://www.w3.org/TR/trace-context/

lib/internal/bootstrap/realm.js

@@ -124,14 +124,15 @@ const legacyWrapperList = new SafeSet([
 // beginning with "internal/".
 // Modules that can only be imported via the node: scheme.
 const schemelessBlockList = new SafeSet([
+  'otel',
   'sea',
   'sqlite',
   'quic',
   'test',
   'test/reporters',
 ]);
 // Modules that will only be enabled at run time.
-const experimentalModuleList = new SafeSet(['sqlite', 'quic']);
+const experimentalModuleList = new SafeSet(['otel', 'sqlite', 'quic']);
 
 // Set up process.binding() and process._linkedBinding().
 {