[JS] Explore and document workarounds or (better yet) fix Next.js build warnings

[mbleigh]

Describe the bug
With a default Next.js app Genkit causes tons of warnings to be displayed at build time. While it does work, this is ugly and potentially worrisome for users. We should investigate and fix.

> next-genkit@0.1.0 build /Users/bleigh/sandbox/next-genkit
> next build

   ▲ Next.js 15.3.4

   Creating an optimized production build ...
 ⚠ Compiled with warnings in 2000ms

./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/TracerProviderWithEnvExporter.js
Module not found: Can't resolve '@opentelemetry/exporter-jaeger' in '/Users/bleigh/sandbox/next-genkit/node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src'

Import trace for requested module:
./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/TracerProviderWithEnvExporter.js
./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/sdk.js
./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/index.js
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
require.extensions is not supported by webpack. Use a loader instead.

Import trace for requested module:
./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
./node_modules/.pnpm/dotprompt@1.1.1/node_modules/dotprompt/dist/index.js
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
require.extensions is not supported by webpack. Use a loader instead.

Import trace for requested module:
./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
./node_modules/.pnpm/dotprompt@1.1.1/node_modules/dotprompt/dist/index.js
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
require.extensions is not supported by webpack. Use a loader instead.

Import trace for requested module:
./node_modules/.pnpm/handlebars@4.7.8/node_modules/handlebars/lib/index.js
./node_modules/.pnpm/dotprompt@1.1.1/node_modules/dotprompt/dist/index.js
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/registry.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

./node_modules/.pnpm/require-in-the-middle@7.5.2/node_modules/require-in-the-middle/index.js
Critical dependency: require function is used in a way in which dependencies cannot be statically extracted

Import trace for requested module:
./node_modules/.pnpm/require-in-the-middle@7.5.2/node_modules/require-in-the-middle/index.js
./node_modules/.pnpm/@opentelemetry+instrumentation@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/instrumentation/build/esm/platform/node/instrumentation.js
./node_modules/.pnpm/@opentelemetry+instrumentation@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/instrumentation/build/esm/platform/node/index.js
./node_modules/.pnpm/@opentelemetry+instrumentation@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/instrumentation/build/esm/platform/index.js
./node_modules/.pnpm/@opentelemetry+instrumentation@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/instrumentation/build/esm/index.js
./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/sdk.js
./node_modules/.pnpm/@opentelemetry+sdk-node@0.52.1_@opentelemetry+api@1.9.0/node_modules/@opentelemetry/sdk-node/build/src/index.js
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/tracing.js
Module not found: Can't resolve '@genkit-ai/firebase' in '/Users/bleigh/sandbox/next-genkit/node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib'

Import trace for requested module:
./node_modules/.pnpm/@genkit-ai+core@1.14.0/node_modules/@genkit-ai/core/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/tracing.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/genkit.js
./node_modules/.pnpm/genkit@1.14.0/node_modules/genkit/lib/index.mjs
./src/app/api/route.ts

 ✓ Compiled successfully in 2000ms
 ✓ Linting and checking validity of types    
 ✓ Collecting page data    
 ✓ Generating static pages (6/6)
Shutting down all Genkit servers...
   Finalizing page optimization  .   Collecting build traces  .Shutting down all Genkit servers...
 ✓ Collecting build traces    
 ✓ Finalizing page optimization    

Route (app)                                 Size  First Load JS    
┌ ○ /                                    5.63 kB         107 kB
├ ○ /_not-found                            973 B         102 kB
└ ƒ /api                                   136 B         101 kB
+ First Load JS shared by all             101 kB
  ├ chunks/664-4a828d82e546d052.js       46.1 kB
  ├ chunks/68fe4062-d779b407d9ceed68.js  53.2 kB
  └ other shared chunks (total)          1.89 kB


○  (Static)   prerendered as static content
ƒ  (Dynamic)  server-rendered on demand

To Reproduce

1. Create a fresh Next.js app with create-next-app
2. npm i genkit @genkit-ai/googleai @genkit-ai/next
3. Create file src/app/api/route.ts with the following content:

import googleAI from "@genkit-ai/googleai";
import { genkit, z } from "genkit";
import appRoute from "@genkit-ai/next";

const ai = genkit({
  plugins: [googleAI()],
});

const testFlow = ai.defineFlow({ name: "foo" }, async () => "bar");
export const POST = appRoute(testFlow);

4. Run npm run build.

Expected behavior
The build completes successfully without spewing warning messages.

Screenshots
If applicable, add screenshots to help explain your problem.

Runtime (please complete the following information):

• OS: [e.g. Linux, MacOS]
• Version [e.g. 22]

** Node version

• run node --version at paste here

Additional context
At the moment I've managed to hack together this Next.js config file that fixes the build warnings for the default settings, but I have not yet confirmed that everything still works with this config. Need to at a minimum test that telemetry and Dotprompt still work from the built artifacts here:

import type { NextConfig } from "next";

const nextConfig: NextConfig = {
  webpack(config) {
    config.externals = [
      ...(config.externals || []),
      "@opentelemetry/sdk-node",
      "@genkit-ai/firebase",
    ];
    config.resolve.alias = {
      ...config.resolve.alias,
      "handlebars/runtime": "handlebars/dist/cjs/handlebars.runtime",
      handlebars: "handlebars/dist/cjs/handlebars",
    };

    return config;
  },
  outputFileTracingRoot: "../../",
};

export default nextConfig;

Make sure to verify Turbopack as well.