How to get last write date of a RavenDB collection/index

I want to detect when a collection or index was last modified or written to.

Note: This works on document level, but i need on collection/index level. How to get last write date of a RavenDB document via C#

RavenQueryStatistics stats;

using(var session = _documentStore.OpenSession()) {
    session.Query<MyDocument>()
        .Statistics(out stats)
        .Take(0) // don't load any documents as we need only the stats
        .ToArray(); // this is needed to trigger server side query execution
}

DateTime indexTimestamp = stats.IndexTimestamp;
string indexEtag = stats.IndexEtag;;