feat(instrumentation-undici): add custom metric labels

Fixes #2597

---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/open-telemetry/opentelemetry-js-contrib/issues/2597?shareId=XXXX-XXXX-XXXX-XXXX).

Author: lionskape

plugins/node/instrumentation-undici/README.md

@@ -57,6 +57,7 @@ Undici instrumentation has few options available to choose from. You can set the
 | [`startSpanHook`](https://github.com/open-telemetry/opentelemetry-js-contrib/blob/main/plugins/node/instrumentation-undici/src/types.ts#L79)           | `StartSpanHookFunction` | Function for adding custom attributes before a span is started.                                                                                                                                                                                              |
 | [`requireParentforSpans`](https://github.com/open-telemetry/opentelemetry-js-contrib/blob/main/plugins/node/instrumentation-undici/src/types.ts#L81)   | `Boolean`               | Require a parent span is present to create new span for outgoing requests.                                                                                                                                                                                   |
 | [`headersToSpanAttributes`](https://github.com/open-telemetry/opentelemetry-js-contrib/blob/main/plugins/node/instrumentation-undici/src/types.ts#L83) | `Object`                | List of case insensitive HTTP headers to convert to span attributes. Headers will be converted to span attributes in the form of `http.{request\|response}.header.header-name` where the name is only lowercased, e.g. `http.response.header.content-length` |
+| [`requestMetricAttributesHook`](https://github.com/open-telemetry/opentelemetry-js-contrib/blob/main/plugins/node/instrumentation-undici/src/types.ts#L85) | `RequestMetricAttributesHookFunction` | Function for adding custom attributes to metrics based on request. |
 
 ### Observations
 

plugins/node/instrumentation-undici/src/types.ts

@@ -63,6 +63,12 @@ export interface StartSpanHookFunction<T = UndiciRequest> {
   (request: T): Attributes;
 }
 
+export interface RequestMetricAttributesHookFunction<
+  RequestType = UndiciRequest
+> {
+  (request: RequestType): Attributes;
+}
+
 // This package will instrument HTTP requests made through `undici` or  `fetch` global API
 // so it seems logical to have similar options than the HTTP instrumentation
 export interface UndiciInstrumentationConfig<
@@ -84,4 +90,6 @@ export interface UndiciInstrumentationConfig<
     requestHeaders?: string[];
     responseHeaders?: string[];
   };
+  /** Function for adding custom attributes to metrics based on request */
+  requestMetricAttributesHook?: RequestMetricAttributesHookFunction<RequestType>;
 }

plugins/node/instrumentation-undici/src/undici.ts

@@ -430,7 +430,7 @@ export class UndiciInstrumentation extends InstrumentationBase<UndiciInstrumenta
     this._recordFromReq.delete(request);
 
     // Record metrics
-    this.recordRequestDuration(attributes, startTime);
+    this.recordRequestDuration(attributes, startTime, request);
   }
 
   // This is the event we get when something is wrong in the request like
@@ -464,10 +464,10 @@ export class UndiciInstrumentation extends InstrumentationBase<UndiciInstrumenta
 
     // Record metrics (with the error)
     attributes[SemanticAttributes.ERROR_TYPE] = error.message;
-    this.recordRequestDuration(attributes, startTime);
+    this.recordRequestDuration(attributes, startTime, request);
   }
 
-  private recordRequestDuration(attributes: Attributes, startTime: HrTime) {
+  private recordRequestDuration(attributes: Attributes, startTime: HrTime, request: UndiciRequest) {
     // Time to record metrics
     const metricsAttributes: Attributes = {};
     // Get the attribs already in span attributes
@@ -485,6 +485,19 @@ export class UndiciInstrumentation extends InstrumentationBase<UndiciInstrumenta
       }
     });
 
+    // Apply custom attributes from the requestMetricAttributesHook if defined
+    const config = this.getConfig();
+    const customAttributes = safeExecuteInTheMiddle(
+      () => config.requestMetricAttributesHook?.(request),
+      e => e && this._diag.error('caught requestMetricAttributesHook error: ', e),
+      true
+    );
+    if (customAttributes) {
+      Object.entries(customAttributes).forEach(([key, val]) => {
+        metricsAttributes[key] = val;
+      });
+    }
+
     // Take the duration and record it
     const durationSeconds =
       hrTimeToMilliseconds(hrTimeDuration(startTime, hrTime())) / 1000;