Update Picklist in Salesforce using Metadata API

ClassA.cls

MetadataApiService.MetadataPort service = createService();

MetadataApiService.CustomField customField = (MetadataApiService.CustomField) service.readMetadata('CustomField', new String[]{
        'Account.Demo__c'
}).getRecords()[0];

MetadataApiService.GlobalValueSet globalValueSet = (MetadataApiService.GlobalValueSet) service.readMetadata('GlobalValueSet',
        new List<String>{
                customField.valueSet.valueSetName
        }).getRecords()[0];

List<MetadataApiService.CustomValue> customValues = new List<MetadataApiService.CustomValue>();
for (CustType__c custType : relevantCustTypes) {
    MetadataApiService.CustomValue customValue = globalValueSet.customValue[0].clone();
    customValue.label = custType.Name;
    customValue.fullName = custType.Name;
    customValues.add(customValue);
}

globalValueSet.customValue.addAll(customValues);

List<MetadataApiService.Metadata> metadataList = new List<MetadataApiService.Metadata>();
metadataList.add(globalValueSet);

List<MetadataApiService.SaveResult> saveResults = service.updateMetadata(metadataList);
for (MetadataApiService.SaveResult saveResult : saveResults) {
    if (saveResult.success) {
        System.debug('Successfully updated');
    } else {
        if (saveResult.errors.size() > 0) {
            System.debug('Error : ' + saveResult.errors[0].message);
            throw new MetadataServiceExamplesException('Error : ' + saveResult.errors[0].message);
        }
    }
}
        
        
        
public static MetadataApiService.MetadataPort createService() {
    MetadataApiService.MetadataPort service = new MetadataApiService.MetadataPort();
    service.AllOrNoneHeader = new MetadataApiService.AllOrNoneHeader_element();
    service.AllOrNoneHeader.allOrNone = true;
    service.SessionHeader = new MetadataApiService.SessionHeader_element();
    service.SessionHeader.sessionId = UserInfo.getSessionId();
    service.timeout_x = 120000;
    return service;
}

private class MetadataServiceExamplesException extends Exception {
}