Annotations: introduce path segment to address the count of a collection: We could use /$count or @odata.count.
The first is consistent with URL rules, the second with odata.mediaContentType etc. that we introduced recently in ODATA-227.

<?xml version="1.0" encoding="utf-8"?>
<edmx:Edmx Version="4.0" xmlns:edmx="http://docs.oasis-open.org/odata/ns/edmx">
<edmx:Reference Url="http://services.odata.org/V3/OData/OData.svc/$metadata" />
<edmx:DataServices>
<Schema xmlns="http://docs.oasis-open.org/odata/ns/edm" Namespace="CountSample" Alias="CS">

<Term Name="MostImportantDataFields" Type="Collection(CS.DataFieldType)" />
<ComplexType Name="DataFieldType">
<Property Name="Label" Type="String" Nullable="true" />
<Property Name="Value" Type="Edm.PrimitiveType" Nullable="false" />
<Property Name="NavigationPath" Type="Edm.NavigationPropertyPath" Nullable="true" />
</ComplexType>

<Annotations Target="ODataDemo.Category">
<Annotation Term="CS.MostImportantDataFields">
<Collection>
<Record>
<PropertyValue Property="Label" String="Category Name" />
<PropertyValue Property="Value" Path="Name" />
</Record>
<Record>
<PropertyValue Property="Label" String="Numbers of Products related to this Category" />
<!-- Alternative 1 -->
<PropertyValue Property="Value" Path="Products/$count" /> <!-- see $orderby and $filter -->
<!-- Alternative 2 -->
<PropertyValue Property="Value" Path="Products/@odata.count" /><!-- see CSDL 16.2.10 The edm:Path Expression -->
<PropertyValue Property="NavigationPath" NavigationPropertyPath="Products" />
</Record>
</Collection>
</Annotation>
</Annotations>
</Schema>
</edmx:DataServices>
</edmx:Edmx>