Minimal Repro of BatchHTTPClient not functioning correctly

Datadog RUM attaches headers to outgoing fetch and XHR requests to be able to link together traces. When using Apollo's HTTPLink, this functions correctly, but it doesn't with BatchHTTPLink.

This was previously reported as an issue and fixed for HTTPLink in PR #8603 but the same fix needs to be applied to BatchHTTPLink.

Repo Setup

# Assuming you have nvm installed, if not skip this step or install the latest node version
nvm install 
corepack enable
yarn install
yarn start

Repro

Error Case

Open localhost:9000 in a browser
open the network tab
You should see a request to http://localhost:9000/graphql
Examine the request headers for the request and see that there are no x-datadog- headers attached.

Working Case

Change the client passed to the ApolloProvider in app/Component.js to use unbatchedClient
Refresh the browser
Examine the request headers again for the /graphql request
There are now several x-datadog- headers attached to the request

Workaround Hack

By passing in a custom function for fetch which forces the fetch to be reevaluated, we can get around the issues with BatchHTTPClient.