public PortsMeta tag(String meta) {
        if (currentOffset == null) {
            throw new NullPointerException("No current port offset to tag, use 'on#1'");
        }
        return register(currentOffset, meta);
    }
Tag a previously setup port (using 'on') with the specified tag.

@param meta the tag to apply to the current port
@return this portsmeta, to allow further .tag calls
config-model/src/main/java/com/yahoo/vespa/model/PortsMeta.java