docs(configuration): pair threads.preload with preloadRequire for dd-trace

`dd-trace/register.js` only installs the ESM loader hooks; it never calls
`init()`. Configuring `threads.preload: dd-trace/register.js` alone therefore
leaves the tracer uninitialized - spans are created and carry plausible trace
ids, but they are no-ops and nothing is exported, so the failure is silent.
`dd-trace/init` (`threads.preloadRequire`) is the entry that starts the tracer.

Replace the claim that `dd-trace/init` "only covers the main thread", show both
keys together in the dd-trace example, and cross-reference the two bullets.
Behavior is scoped to dd-trace 6.x, since a future major could change
`register.js`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Ethan-Arrowood

reference/configuration/options.md

@@ -65,11 +65,12 @@ threads:
 - `maxHeapMemory` — Heap limit per thread (MB)
 - `heapSnapshotNearLimit` — Write a `.heapsnapshot` file when a thread nears its heap limit (loadable in Chrome DevTools Memory tab); _Default_: `false`. See [Worker Thread Debugging](./debugging.md#heap-snapshots-near-the-limit)
 - `debug` — Enable Node.js inspector; sub-options: `port`, `startingPort`, `host`, `waitForDebugger`. See [Worker Thread Debugging](./debugging.md)
-- `preload` <VersionBadge version="v5.2.0" /> — Module, or list of modules, to load (via Node's `--import`) before any Harper or application module on each worker thread. Intended for instrumentation/APM agents that must load first to instrument subsequent module loads. Use the agent's ESM/register entry — e.g. `dd-trace/register.js`, which registers the loader hooks that instrument worker threads (where Harper runs its work); the plain `dd-trace/init` (`--require`) entry only covers the main thread. Bare specifiers resolve against the `node_modules` of your installed [components](../components/overview.md) — so the agent can be shipped as a dependency of a deployed component — and absolute paths are also accepted. Applies to worker threads only (not under Bun).
+- `preload` <VersionBadge version="v5.2.0" /> — Module, or list of modules, to load (via Node's `--import`) before any Harper or application module on each worker thread. Intended for instrumentation/APM agents that must load first to instrument subsequent module loads. Use the agent's ESM/register entry — e.g. `dd-trace/register.js`, which installs the ESM loader hooks that produce automatic instrumentation for `import`-loaded modules. As measured on dd-trace 6.x, that entry only registers the loader hooks and never calls `init()`, so `preload` on its own leaves the tracer uninitialized: it still hands out spans with plausible trace ids, but they are no-ops and nothing is ever exported. Pair it with `preloadRequire: dd-trace/init`, which is the entry that actually starts the tracer. Bare specifiers resolve against the `node_modules` of your installed [components](../components/overview.md) — so the agent can be shipped as a dependency of a deployed component — and absolute paths are also accepted. Applies to worker threads only (not under Bun).
 
 ```yaml
 threads:
-  preload: dd-trace/register.js
+  preloadRequire: dd-trace/init # starts the tracer
+  preload: dd-trace/register.js # ESM loader hooks for automatic instrumentation
 ```
 
 Or several modules:
@@ -81,11 +82,11 @@ threads:
     - /opt/instrumentation/agent.mjs
 ```
 
-- `preloadRequire` <VersionBadge version="v5.2.0" /> — Same as `preload`, but loads modules via Node's `--require` (CommonJS) instead of `--import`. Use this for agents that document the `--require` path and do not need ESM loader hooks (e.g. `dd-trace/init`, Dynatrace OneAgent). Same resolution rules as `preload`.
+- `preloadRequire` <VersionBadge version="v5.2.0" /> — Same as `preload`, but loads modules via Node's `--require` (CommonJS) instead of `--import`. Use this for agents that document the `--require` path (e.g. `dd-trace/init`, Dynatrace OneAgent). Same resolution rules as `preload`. For dd-trace, `dd-trace/init` is the entry that starts the tracer, and it does not register the ESM loader hooks — keep `preload: dd-trace/register.js` alongside it, as shown under `preload` above.
 
 ```yaml
 threads:
-  preloadRequire: dd-trace/init
+  preloadRequire: dd-trace/init # starts the tracer; pair with preload (see above)
 ```
 
 ---