Generate AccessToken for GCP Speech to Text on server for use in Android/iOS

Sorry for the delay, I was able to get it all to work together and am now only circling back to post an extremely simplified how-to. To start, I installed the following library on the server endpoint project https://www.npmjs.com/package/google-auth-library

The server endpoint in this case is lacking any authentication/authorization etc for simplicity's sake. I'll leave that part up to you. We are also going to pretend this endpoint is reachable from https://www.example.com/token

The expectation being, calling https://www.example.com/token will result in a response with a string token, a number for expires, and some extra info about how the token was generated:

ie:

{"token":"sometoken", "expires":1234567, "info": {... additional stuff}}

Also for this example I used a ServiceAccountKey file which will be stored on the server, The suggested route is to set up a server environment variable and use https://cloud.google.com/docs/authentication/production#finding_credentials_automatically however this is for the examples sake, and is easy enough for a quick test. These files look something like the following: ( honor system don't steal my private key )

ServiceAccountKey.json

{
  "type": "service_account",
  "project_id": "project-id",
  "private_key_id": "378329234klnfgdjknfdgh9fgd98fgduiph",
  "private_key": "-----BEGIN PRIVATE KEY-----\nThisIsTotallyARealPrivateKeyPleaseDontStealIt=\n-----END PRIVATE KEY-----\n",
  "client_email": "project-id@appspot.gserviceaccount.com",
  "client_id": "12345678901234567890",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/project-id%40appspot.gserviceaccount.com"
}

So here it is a simple endpoint that spits out an AccessToken and a number indicating when the token expires (so you can call for a new one later).

endpoint.js

const express = require("express");
const auth = require("google-auth-library");
const serviceAccount = require("./ServiceAccountKey.json");

const googleauthoptions = {
    scopes: ['https://www.googleapis.com/auth/cloud-platform'],
    credentials: serviceAccount
};

const app = express();
const port = 3000;
const auth = new auth.GoogleAuth(googleauthoptions);
auth.getClient().then(client => {
    app.get('/token', (req, res) => {
        client
            .getAccessToken()
            .then((clientresponse) => {
            if (clientresponse.token) {
                return clientresponse.token;
            }
            return Promise.reject('unable to generate an access token.');
        })
            .then((token) => {
            return client.getTokenInfo(token).then(info => {
                const expires = info.expiry_date;
                return res.status(200).send({ token, expires, info });
            });
        })
            .catch((reason) => {
            console.log('error:  ' + reason);
            res.status(500).send({ error: reason });
        });
    });
    app.listen(port, () => {
        console.log(`Server is listening on https://www.example.com:${port}`);
    });
    return;
});

Almost done now, will use android as an example. First clip will be how it was originally pulling from device file:

public static final List<String> SCOPE = Collections.singletonList("https://www.googleapis.com/auth/cloud-platform");
    final GoogleCredentials credentials = GoogleCredentials.fromStream(this.mContext.getResources().openRawResource(R.raw.credential)).createScoped(SCOPE);
      final AccessToken token = credentials.refreshAccessToken();
final string token = accesstoken.getTokenValue();
final long expires = accesstoken.getExpirationTime().getTime()
final SharedPreferences prefs = getSharedPreferences(PREFS, Context.MODE_PRIVATE);
    prefs.edit().putString(PREF_ACCESS_TOKEN_VALUE, value).putLong(PREF_ACCESS_TOKEN_EXPIRATION_TIME, expires).apply();
    fetchAccessToken();

Now we got our token from the endpoint over the internet (not shown), with token and expires information in hand, we handle it in the same manner as if it was generated on the device:

//
// lets pretend endpoint contains the results from our internet request against www.example.com/token
final string token = endpoint.token;
final long expires = endpoint.expires
    final SharedPreferences prefs = getSharedPreferences(PREFS, Context.MODE_PRIVATE);
    prefs.edit().putString(PREF_ACCESS_TOKEN_VALUE, value).putLong(PREF_ACCESS_TOKEN_EXPIRATION_TIME, expires).apply();
    fetchAccessToken();

Anyway hopefully that is helpful if anyone has a similar need.