UpdateGatewayMetadata sets a gateway's card WITHOUT touching its stake.

MsgUpdateGatewayMetadata sets a staked gateway's card.

Deliberately separate from MsgStakeGateway, which enforces a strictly-positive stake delta on every call -- folding metadata into it would mean escrowing real POKT to fix a typo. A freshly staked gateway starts with no card and becomes descriptor-complete via a follow-up call.

Full replace: the supplied metadata replaces whatever is stored. Omitting it (nil) leaves the stored card untouched, matching MsgAddService's behaviour for services.