Enable metadata in ajax enabled WCF web service

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C#

WCF

C#4.0

ASP.NET4

Hi, I'm working on a ASP.Net project and It contains a AJAX-enabled WCF service. when I run that service always I get a message saying "Metadata publishing for this service is currently disabled." So I tried to enable the service by editing the web.config file but i couldn't do it. I don't know much about WCF services. since this is a ASP.Net project I don't have a app.config file. so I need a little help to enable this metadata thing to run my web service. thanks. my web.config file is given below.

<?xml version="1.0"?>
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
<system.serviceModel>
<services>
<!-- Before deployment, you should remove the returnFaults behavior configuration to avoid disclosing information in exception messages -->
<service name="ImageService" behaviorConfiguration="returnFaults">
<endpoint contract="ImageService" binding="mexHttpBinding" address="mex"/>
</service>
</services>
<behaviors>
<serviceBehaviors>
<behavior name="returnFaults" >
<serviceDebug includeExceptionDetailInFaults="true" />
<serviceMetadata httpGetEnabled="true" />
</behavior>
</serviceBehaviors>
</behaviors>
</system.serviceModel>
<system.web>
<compilation debug="true"/>
</system.web>
</configuration>