fix(koa): ignore generator-based Koa middleware

In Koa 2.x, which is the Koa version currently targeted by the
instrumentation, generator-based middleware from Koa 0.x and 1.x
are deprecated, and a deprecation warning is shown when these
middleware are used.

However, most of these middleware still work as intended on Koa
version 2.x, due to the use of the `koa-convert` compatibility
layer, which is applied automatically when a generator-based
middleware is used. Because of this, low-quality tutorial websites
continue to recommend the usage of generator-based middleware for
error handling.

Before this change, using a generator-based middleware alongside
the OpenTelemetry instrumentation would break the request, as the
instrumentation does not expect a generator-based middleware.
Furthermore, the replacement of the middleware with its
async-based patch would silence the deprecation warning about
the generator-based middleware.

After this change, generator-based middleware are not
instrumented, and the deprecation warning is shown. Users who wish
for their generator-based middleware to be instrumented can
manually use `koa-convert`.

In Koa 3.x, generator-based middleware will not work, with or
without instrumentation.

plugins/node/opentelemetry-instrumentation-koa/README.md

@@ -38,7 +38,9 @@ registerInstrumentations({
 });
 ```
 
-See [`examples/koa`](https://github.com/open-telemetry/opentelemetry-js-contrib/tree/main/examples/koa) for a short example using both Koa and @koa/router
+See [`examples/koa`](https://github.com/open-telemetry/opentelemetry-js-contrib/tree/main/examples/koa) for a short example using both Koa and @koa/router.
+
+Note that generator-based middleware are deprecated and won't be instrumented.
 
 ### Koa Instrumentation Options
 

plugins/node/opentelemetry-instrumentation-koa/src/instrumentation.ts

@@ -134,6 +134,13 @@ export class KoaInstrumentation extends InstrumentationBase<typeof koa> {
     )
       return middlewareLayer;
     middlewareLayer[kLayerPatched] = true;
+    if (
+      middlewareLayer.constructor.name === "GeneratorFunction" ||
+      middlewareLayer.constructor.name === "AsyncGeneratorFunction"
+    ) {
+      api.diag.debug('ignoring generator-based Koa middleware layer');
+      return middlewareLayer;
+    }
     api.diag.debug('patching Koa middleware layer');
     return async (context: KoaContext, next: koa.Next) => {
       const parent = api.trace.getSpan(api.context.active());