ref(node): Remove two Node <20 leftovers (hrtime.bigint, undici channel refs)

Both were guarded by a condition that has since passed: the engine floor
moved to Node >=20.19.0 in 4f03438.

- anr/worker.ts: `createHrTimer()` now uses `process.hrtime.bigint()`; the
  TODO asking for it was conditioned on dropping Node 8.
- node-fetch/undici-instrumentation.ts: `_channelSubs` only existed to keep
  a reference alive around nodejs/node#42170, which needed Node 18.18.0
  support. Replaced with the `_isInstrumented` boolean its own comment
  proposed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JGK2ap3HHAy2WuDwoS8yTW

Author: wittachai-as

packages/node/src/integrations/anr/worker.ts

@@ -286,16 +286,14 @@ if (options.captureStackTrace) {
 }
 
 function createHrTimer(): { getTimeMs: () => number; reset: VoidFunction } {
-  // TODO (v8): We can use process.hrtime.bigint() after we drop node v8
-  let lastPoll = process.hrtime();
+  let lastPoll = process.hrtime.bigint();
 
   return {
     getTimeMs: (): number => {
-      const [seconds, nanoSeconds] = process.hrtime(lastPoll);
-      return Math.floor(seconds * 1e3 + nanoSeconds / 1e6);
+      return Number((process.hrtime.bigint() - lastPoll) / 1_000_000n);
     },
     reset: (): void => {
-      lastPoll = process.hrtime();
+      lastPoll = process.hrtime.bigint();
     },
   };
 }

packages/node/src/integrations/node-fetch/undici-instrumentation.ts

@@ -74,9 +74,7 @@ import type {
 // `http.request.method_original` is not part of `@sentry/conventions`, so we keep it inline.
 const ATTR_HTTP_REQUEST_METHOD_ORIGINAL = 'http.request.method_original';
 
-// Keep ref to avoid https://github.com/nodejs/node/issues/42170 bug
-// We can replace this with _isInstrumented once we drop support for Node.js 18.18.0
-const _channelSubs: Array<unknown> = [];
+let _isInstrumented = false;
 const spanFromReq = new WeakMap<UndiciRequest, Span>();
 // Whether breadcrumbs (and span-less trace propagation) should be skipped for a given request.
 // We evaluate this at request-creation time because the active context is no longer correct by the
@@ -101,9 +99,10 @@ const propagationDecisionMap = new LRUMap<string, boolean>(100);
  */
 export function instrumentUndici(config: NodeFetchOptions = {}): void {
   // Avoid duplicate subscriptions
-  if (_channelSubs.length) {
+  if (_isInstrumented) {
     return;
   }
+  _isInstrumented = true;
 
   subscribeToChannel('undici:request:create', message => onRequestCreated(config, message as RequestMessage));
   subscribeToChannel('undici:client:sendHeaders', message =>
@@ -118,7 +117,7 @@ function subscribeToChannel(
   diagnosticChannel: string,
   onMessage: (message: unknown, name: string | symbol) => void,
 ): void {
-  _channelSubs.push(diagch.subscribe?.(diagnosticChannel, onMessage));
+  diagch.subscribe?.(diagnosticChannel, onMessage);
 }
 
 function parseRequestHeaders(request: UndiciRequest): Map<string, string | string[]> {