docs: add allowCustomFetcher config flag documentation for TypeScript SDK

fern/products/sdks/overview/typescript/configuration.mdx

@@ -16,6 +16,65 @@ groups:
           noSerdeLayer: false
 ```
 
+<ParamField path="allowCustomFetcher" type="boolean" default="false" toc={true}>
+Enables passing a custom fetcher function to the client. The custom fetcher intercepts all SDK HTTP requests and can be used for logging, tracing, metrics, header injection, routing through proxies, custom retries, or other cross-cutting behavior.
+
+```yaml title="generators.yml"
+config:
+  allowCustomFetcher: true
+```
+
+Example using a custom fetcher for request logging:
+
+```typescript title="src/wrapper/loggingFetcher.ts"
+import { fetcher as defaultFetcher, type FetchFunction } from "../core/fetcher";
+
+export function createLoggingFetcher(): FetchFunction {
+  return async (args) => {
+    const start = Date.now();
+    const headers = {
+      ...(args.headers ?? {}),
+      "X-Request-Id": globalThis.crypto?.randomUUID?.() ?? `${Date.now()}`
+    };
+    const res = await defaultFetcher({ ...args, headers });
+    const ms = Date.now() - start;
+    console.log(`${args.method} ${args.url} -> ${res.status} (${ms}ms)`);
+    return res;
+  };
+}
+```
+
+```typescript title="src/wrapper/PlantStoreClient.ts"
+import { PlantStoreClient as FernClient } from "../Client";
+import { createLoggingFetcher } from "./loggingFetcher";
+
+type FernClientOptions = ConstructorParameters<typeof FernClient>[0];
+type Options = Omit<FernClientOptions, "fetcher">;
+
+export class PlantStoreClient extends FernClient {
+  constructor(options: Options) {
+    super({
+      ...options,
+      fetcher: createLoggingFetcher()
+    });
+  }
+}
+```
+
+```typescript
+import { PlantStoreClient } from "plant-store-sdk";
+
+const client = new PlantStoreClient({
+  environment: "https://api.plantstore.com"
+});
+
+await client.plants.get("monstera-123");
+// Logs: GET /plants/monstera-123 -> 200 (42ms)
+```
+
+See [Dynamic authentication](/sdks/generators/typescript/dynamic-authentication) for an example of adding authentication middleware with a custom fetcher.
+</ParamField>
+
 <ParamField path="allowExtraFields" type="boolean" toc={true}>
 Allow fields that are not defined in object schemas. This only applies to serde.
 
@@ -555,4 +614,4 @@ interface ObjectWithLongAndBigInt {
 	printMyString(myString);
 	```
 
-</ParamField>
+</ParamField>