Apex domains cannot have CNAME records.
Question and response
Question
dig metronet.cz. NS
Answer
metronet.cz. 5 NS ns1.metronet.cz.
metronet.cz. 5 NS ns2.metronet.cz.
Authority
The response has no authority records.
Additional
ns1.metronet.cz. 5 A 46.234.123.41
ns2.metronet.cz. 5 A 78.110.208.2
ns1.metronet.cz. 5 AAAA 2a01:28:ca:258:0:0:0:2
ns2.metronet.cz. 5 AAAA 2a02:13bb:0:1:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0