private Callback<Void> securityPostProcessRequestCallback() {
      return buildCallback(frontendMetrics.postAccountsSecurityPostProcessRequestMetrics, securityCheckResult -> {
        RetainingAsyncWritableChannel channel =
            new RetainingAsyncWritableChannel(frontendConfig.maxJsonRequestSizeBytes);
        restRequest.readInto(channel, fetchAccountUpdateBodyCallback(channel));
      }, uri, logger, finalCallback);
    }
After {@link SecurityService#postProcessRequest} finishes, read the request body content.
@return a {@link Callback} to be used with {@link SecurityService#postProcessRequest}.
ambry-frontend/src/main/java/com/github/ambry/frontend/PostAccountsHandler.java