oauth2.googleapis.com API request is not going through proxy

We are using FireBaseSDK(firebase-admin-9.2.0) to push notifications, while testing we noted below.

-- Request going through proxy
POST https://fcm.googleapis.com/v1/projects/

--Request not going through proxy
POST https://oauth2.googleapis.com/token

Kindly let us know why oauth2 request is not going through proxy.

Start by reproducing the proxy behavior with Firebase Admin Java SDK 9.2.0, comparing the FCM request with the POST request to oauth2.googleapis.com/token. Trace the SDK's proxy or HTTP transport configuration and document the cause; done means the OAuth token request follows the configured proxy or the limitation is clearly explained.

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