presto-base-jdbc/src/main/java/io/prestosql/plugin/jdbc/credential/ExtraCredentialProvider.java/ExtraCredentialProvider/getConnectionUser
class ExtraCredentialProvider:    @Override
    public Optional<String> getConnectionUser(Optional<JdbcIdentity> jdbcIdentity)
    {
        if (jdbcIdentity.isPresent() && userCredentialName.isPresent()) {
            Map<String, String> extraCredentials = jdbcIdentity.get().getExtraCredentials();
            if (extraCredentials.containsKey(userCredentialName.get())) {
                return Optional.of(extraCredentials.get(userCredentialName.get()));
            }
        }
        return delegate.getConnectionUser(jdbcIdentity);
    }