Expose index age in ILM explain output (#4142)

addresses elastic/elasticsearch#44457

Author: Mpdreamz

src/Nest/XPack/Ilm/ExplainLifecycle/ExplainLifecycleResponse.cs

@@ -78,6 +78,12 @@ public class LifecycleExplain
 		[DataMember(Name = "step_time_millis")]
 		[JsonFormatter(typeof(DateTimeOffsetEpochMillisecondsFormatter))]
 		public DateTimeOffset StepTime { get; internal set; }
+
+		/// <summary>
+		/// The age of the index, useful to gauge when the next phase will trigger based on <see cref="IPhase.MinimumAge"/>
+		/// </summary>
+		[DataMember(Name = "age")]
+		public Time Age { get; internal set; }
 	}
 
 	public class PhaseExecution

src/Tests/Tests/XPack/Ilm/IlmApiTests.cs

@@ -176,7 +176,7 @@ [I] public async Task IlmExplainLifecycleResponse() => await Assert<ExplainLifec
 
 			indexDict.Index.Should().Be("project");
 			indexDict.Managed.Should().Be(false);
-
+			indexDict.Age.Should().BeNull();
 		});
 
 		[I] public async Task IlmStopResponse() => await Assert<StopIlmResponse>(IlmStopStep, (v, r) =>