Globally register error handling

[amannn]

Is your feature request related to a problem? Please describe.

Currently, error handling is somewhat cumbersome to set up. It needs to be set up both in i18n.ts as well as NextIntlClientProvider to cover both Server- as well as Client Components.

Most configuration from i18n.ts can automatically be inherited by Client Components, but for onError and getMessageFallback this is not the case since functions aren't serializable. Therefore this requires workaround (see e.g. #1226).

Ideally, we'd let users register error handling once and use it across a whole app without workarounds (regardless of RSC/SSR/CSR).

Describe the solution you'd like

If Next.js would allow client-side instrumentation, we could use instrumentation.ts for this use case. This would be the preferred solution from my perspective.

// instrumentation.ts

import {onError, configureMessageFallback, IntlErrorCode} from 'next-intl';

// Called for both `react-server` as well as `react-client`
export async function register() {
  // Called when an error is encountered during formatting
  onError((error) => {
    if (error.code === IntlErrorCode.MISSING_MESSAGE) {
      // Missing translations are expected and should only log an error
      console.error(error);
    } else {
      // Other errors indicate a bug in the app and should be reported
      reportToErrorTracking(error);
    }
  });

  // Called to provide a fallback message when formatting has failed
  configureMessageFallback(({namespace, key, error}) => {
    const path = [namespace, key].filter((part) => part != null).join('.');

    if (error.code === IntlErrorCode.MISSING_MESSAGE) {
      return path + ' (fallback)';
    } else {
      return path + ' (formatting error)';
    }
  });
}

Implementation-wise we'd likely write to a global store if these functions are called in order to register the handler.

Describe alternatives you've considered

Alternatively, we could explore automatically creating an onError server action that can be serialized across the network. This wouldn't work for getMessageFallback though.

(this issue was created after working on a first draft on feat/error-instrumentation)

[amannn]

Seems like there's something cooking on the Next.js side: https://x.com/huozhi/status/1828726448892354741. We should probably wait for this.