Handle token expiration automatically in the MarketingCloudSession class

FWIW this is the code I have in my project for the token expiration

  public function token($forceLogin = FALSE) {
    // @TODO inject this.
    $state = \Drupal::state();
    if ($state->get('unhcr_marketing_cloud_requesting_token') == TRUE) {
      // Wait n seconds to prevent overloading token request with simultaneous
      // requests.
      sleep($this->config
        ->get('requestToken_wait'));
    }

    $token = $state->get('unhcr_marketing_cloud_token');
    $expired = \Drupal::time()->getCurrentTime() >= $state->get('unhcr_marketing_cloud_token_expire');
    $recalculate_token = $forceLogin || empty($token) || $expired;
    if (!$recalculate_token) {
      return $token;
    }

    // Prevent flooding by setting requesting_token to TRUE.
    $state->set('unhcr_marketing_cloud_requesting_token', TRUE);
    $token = FALSE;

    // Validate required params for token request.
    $token_requisites = TRUE;
    $clientId = $this->config->get('client_id');

    if (empty($clientId)) {
      \Drupal::logger(__METHOD__)->error('Bad config data: %missingData', ['%missingData' => 'client_id']);
      $token_requisites = FALSE;
    }
    $clientSecret = $this->config->get('client_secret');
    if (empty($clientSecret)) {
      \Drupal::logger(__METHOD__)->error('Bad config data: %missingData', ['%missingData' => 'client_secret']);
      $token_requisites = FALSE;
    }

    $scope = $this->config->get('scope');
    $accountId = $this->config->get('account_id');
    // Make token request/s.
    if ($token_requisites) {
      $loginAttempts = 0;
      $loginAttemptsMax = $this->config->get('login_attempts_max');
      while (!$token && $loginAttempts++ < $loginAttemptsMax) {
        $token = $this->requestToken($clientId, $clientSecret, $scope, $accountId);
      }
    }

    $state->set('unhcr_marketing_cloud_requesting_token', FALSE);
    $state->set('unhcr_marketing_cloud_token', $token->access_token);
    $state->set('unhcr_marketing_cloud_token_expire', \Drupal::time()->getCurrentTime() + $token->expires_in);

    return $token;
  }

And then on the token method, I return the whole thing:

      $response = \Drupal::httpClient()->post($url, [
        'verify' => FALSE,
        'headers' => ['Content-Type' => 'application/x-www-form-urlencoded'],
        'form_params' => $formParams,
      ]);
      return json_decode($response->getBody());

Needs DI as well, but this handles more or less this issue and #3195684: State API is a better fit to store the token, I can try to make a patch if you're interested

Sounds good, even if the docs say 20 mins, my experience is that MC is a bit... stingy? and never gives you 20 minutes, which was causing edges case on my use case