plone.app.discussion/plone/app/discussion/profiles/default/portal_atct.xml

<?xml version="1.0"?>
<atcttool xmlns:i18n="http://xml.zope.org/namespaces/i18n">
    <topic_indexes i18n:domain="plone">

        <index name="commentators" friendlyName="Commentators"
            description="Users who have commented on the item"
            enabled="True" i18n:attributes="description; friendlyName">

            <criteria>ATCurrentAuthorCriterion</criteria>
            <criteria>ATSimpleStringCriterion</criteria>
            <criteria>ATListCriterion</criteria>

        </index>

        <index
            name="total_comments"
            enabled="False"
            friendlyName="Total number of comments"
            description="Total number of comments on this item."
            i18n:attributes="description; friendlyName">

            <criteria>ATSimpleIntCriterion</criteria>

        </index>

    </topic_indexes>
    <topic_metadata i18n:domain="plone">

        <metadata
            name="total_comments"
            enabled="True"
            friendlyName="Total number of comments"
            description="Total number of comments on this item."
            />

    </topic_metadata>
</atcttool>