How to get the Document response synchronously from MongoClient using reactive library

We’re currently using spring data-mongodb-reactive for change streams API and looking to run couple of admin commands while the springboot application startup. Basically, I want to run a command and execute another one based on the response. How to get the response document from Publisher<Document> commandResult = db.runCommand(bson); into a variable? here is the whole code snippet.

public class InitDB {

    private final MongoClient mongoClient;

    private static final String enableChangeStreams = """
            {modifyChangeStreams: 1,
                    database: "",
                    collection: "",
                    enable: true}
    private static final String isChangeStreamsEnabled = """
            {aggregate: 1,
              pipeline: [{$listChangeStreams: 1},
                         {$match: {$or: [{database: "", collection: ""}]}}

    public InitDB(final MongoClient mongoClient) {
        this.mongoClient = mongoClient;

    public void initializeDB() {

    private void runCommand(final String command) {

        final MongoDatabase db = mongoClient.getDatabase("admin");
        final Document document = Document.parse(command);
        final Bson bson = document.toBsonDocument();
        final Publisher<Document> commandResult = db.runCommand(bson);
        //How to fetch the command result response into a variable via subscribers?
        final SubscriberHelpers.ObservableSubscriber subscriber = new SubscriberHelpers.ObservableSubscriber();