s => s
.Aggregations(a => a
    .Nested("tags", n => n
        .Path(p => p.Tags)
        .Aggregations(aa => aa
            .Terms("tag_names", t => t
                .Field(p => p.Tags.Suffix("name"))
            )
        )
    )
)

new SearchRequest<Project>
{
    Aggregations = new NestedAggregation("tags")
    {
        Path = "tags",
        Aggregations = new TermsAggregation("tag_names")
        {
            Field = "tags.name"
        }
    }
}

{
  "aggs": {
    "tags": {
      "nested": {
        "path": "tags"
      },
      "aggs": {
        "tag_names": {
          "terms": {
            "field": "tags.name"
          }
        }
      }
    }
  }
}

response.ShouldBeValid();
var tags = response.Aggs.Nested("tags");
tags.Should().NotBeNull();
var tagNames = tags.Terms("tag_names");
tagNames.Should().NotBeNull();

foreach(var item in tagNames.Buckets)
{
    item.Key.Should().NotBeNullOrEmpty();
    item.DocCount.Should().BeGreaterThan(0);
}