I have changed the code now it is working.
https://github.com/financialforcedev/apex-mdapi/blob/master/apex-mdapi/src/classes/MetadataService.cls

How to Update Global Picklist through Metadata API
========================================
Class:
=====
public class updateGlobalPickList {
    public static void updateGlobalPicklist (){
        List<String> listSting = new List<String>{'1','2','3','4','5'};
        MetadataService.MetadataPort service =  MetadataServiceExamples.createService();
        MetadataService.GlobalValueSet GlobalValueSet = new MetadataService.GlobalValueSet();
        MetadataService.ValueSetValuesDefinition valueSetValuesDefinition = new MetadataService.ValueSetValuesDefinition();
        metadataservice.valueSet pt = new metadataservice.valueSet();
        pt.valueSetName = 'CampaignAdvertiser';
        List<MetadataService.CustomValue> mcList = new List<MetadataService.CustomValue>();
        for(string s : listSting){
        MetadataService.CustomValue mc = new MetadataService.CustomValue();
        mc.fullName = s;
        mc.default_x = false;
        mc.isActive = true;
        mc.label = s;
        mcList.add(mc);
        }
        GlobalValueSet.fullName = 'CampaignAdvertiser';
        GlobalValueSet.masterLabel = 'CampaignAdvertiser';
        GlobalValueSet.sorted = false;
        GlobalValueSet.customValue = mcList;
        valueSetValuesDefinition.value = mcList;
        valueSetValuesDefinition.sorted = false;
        pt.valueSetDefinition = valueSetValuesDefinition;
        List<MetadataService.SaveResult> results =
            service.updateMetadata(
                new MetadataService.Metadata[] { GlobalValueSet });
    }
}
Test Class
=========
@istest
public class updateGlobalPickListTest {
    private class WebServiceMockImpl implements WebServiceMock 
    {
        public void doInvoke(
            Object stub, Object request, Map<String, Object> response,
            String endpoint, String soapAction, String requestName,
            String responseNS, String responseName, String responseType) 
        {   
            if(request instanceof  MetadataService.updateMetadata_element)
                response.put('response_x', new MetadataService.updateMetadataResponse_element());
            return;
        }
    } 
    
    @IsTest
    private static void coverGeneratedCodeCRUDOperations()
    {	
        // Null Web Service mock implementation
        System.Test.setMock(WebServiceMock.class, new WebServiceMockImpl());
        // Only required to workaround a current code coverage bug in the platform
        MetadataService metaDataService = new MetadataService();
        // Invoke operations     
        Test.startTest();    
        MetadataService.MetadataPort metaDataPort = new MetadataService.MetadataPort();
        Test.stopTest();
    }
    @IsTest
    private static void coverGeneratedCodeFileBasedOperations1()
    {    	
        // Null Web Service mock implementation
        System.Test.setMock(WebServiceMock.class, new WebServiceMockImpl());
        // Only required to workaround a current code coverage bug in the platform
        MetadataService metaDataService = new MetadataService();
        // Invoke operations    
        Test.startTest();     
        MetadataService.MetadataPort metaDataPort = new MetadataService.MetadataPort();
        metaDataPort.updateMetadata(null);
        Test.stopTest();
    }
    @IsTest
    private static void coverGeneratedCodeTypes()
    {             
        Test.startTest();
        System.Test.setMock(WebServiceMock.class, new WebServiceMockImpl());
        new MetadataService();
        new MetadataService.createMetadataResponse_element();
        new MetadataService.DebuggingInfo_element();
        new MetadataService.LogInfo();
        new MetadataService.DebuggingHeader_element();
        new MetadataService.CallOptions_element();
        new MetadataService.AllOrNoneHeader_element();
        new MetadataService.ExtendedErrorDetails();
        new MetadataService.DescribeMetadataObject();
        new MetadataService.DescribeMetadataResult();
        new MetadataService.Error();
        new MetadataService.SaveResult();
        new MetadataService.GlobalPicklistValue();
        updateGlobalPickList.updateGlobalPicklist();
        //Database.executeBatch(new LeadListViewCreationBatch());
        Test.stopTest();
    }
    /* public static testMethod void testschedule() {
Test.StartTest();
LeadListViewCreationBatchSchedule testsche = new LeadListViewCreationBatchSchedule();
String sch = '0 0 10 * * ?';
system.schedule('Test status Check', sch, testsche );
Test.stopTest();
}*/
}