[Backport 7.6] Add flattened usage field count (#4413)

Relates: #4341, elastic/elasticsearch#48972

Co-authored-by: Russ Cam <russ.cam@elastic.co>

src/Nest/XPack/Info/XPackUsage/XPackUsageResponse.cs

@@ -52,7 +52,7 @@ public class XPackUsageResponse : ResponseBase
 		public XPackUsage DataFrame { get; internal set; }
 
 		[DataMember(Name = "flattened")]
-		public XPackUsage Flattened { get; internal set; }
+		public FlattenedUsage Flattened { get; internal set; }
 
 		[DataMember(Name = "data_science")]
 		public XPackUsage DataScience { get; internal set; }
@@ -148,6 +148,15 @@ public class XPackUsage
 		public bool Enabled { get; internal set; }
 	}
 
+	public class FlattenedUsage : XPackUsage
+	{
+		/// <summary>
+		/// Available in Elasticsearch 7.6.0+
+		/// </summary>
+		[DataMember(Name = "field_count")]
+		public int? FieldCount { get; internal set; }
+	}
+
 	public class VectorUsage : XPackUsage
 	{
 		[DataMember(Name = "dense_vector_fields_count")]

tests/Tests/XPack/Info/XPackInfoApiTests.cs

@@ -117,6 +117,9 @@ [I] public async Task XPackUsageResponse() => await Assert<XPackUsageResponse>(X
 #pragma warning disable 618
 					r.DataFrame.Should().NotBeNull();
 #pragma warning restore 618
+
+				if (TestConfiguration.Instance.InRange(">=7.6.0"))
+					r.Flattened.FieldCount.Should().HaveValue();
 			}
 
 			if (TestConfiguration.Instance.InRange(">=7.5.0"))