RE: Entity wise User Statistics Report using FetchXML and SSRS for Dynamics 365 Online Version
You can do this solely in FetchXML using aggregates. Here's an example query to count the created leads and opportunities
<fetch aggregate='true'>
<entity name='systemuser' >
<attribute name='fullname' alias='createdby' groupby='true' />
<link-entity link-type='outer' name='opportunity' from='createdby' to='systemuserid' >
<attribute name='opportunityid' alias='opportunity_count' aggregate='countcolumn' />
</link-entity>
<link-entity link-type='outer' name='lead' from='createdby' to='systemuserid' >
<attribute name='leadid' alias='lead_count' aggregate='countcolumn' />
</link-entity>
</entity>
</fetch>
This creates a separate field for each entity count, so in the report you'd have one Row Group on the User, then a separate row for each entity. One drawback of this is that, if you add an extra entity to the query, you'll have to add an extra row to the report
Another option could be to make use of activity feeds. If you configure this to create a post each time a record is created, then you could query the post entity, and use the regardingobjectid and createdonbehalfby columns:
<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false">
<entity name="post">
<attribute name="postid" />
<attribute name="createdon" />
<attribute name="createdonbehalfby" />
<attribute name="text" />
<attribute name="regardingobjectid" />
</entity>
</fetch>