Fix draining response in metrics test (#93379)

Author: ManickaP

src/libraries/System.Net.Http/tests/FunctionalTests/MetricsTest.cs

@@ -30,7 +30,7 @@ protected static class InstrumentNames
             public const string ConnectionDuration = "http.client.connection.duration";
             public const string TimeInQueue = "http.client.request.time_in_queue";
         }
-        
+
         protected HttpMetricsTestBase(ITestOutputHelper output) : base(output)
         {
         }
@@ -347,7 +347,7 @@ public Task RequestDuration_CustomTags_Recorded()
                 {
                     ctx.AddCustomTag("route", "/test");
                 });
-                
+
                 using HttpResponseMessage response = await SendAsync(client, request);
 
                 Measurement<double> m = Assert.Single(recorder.GetMeasurements());
@@ -455,6 +455,21 @@ await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>
                 using InstrumentRecorder<double> recorder = SetupInstrumentRecorder<double>(InstrumentNames.RequestDuration);
                 using HttpRequestMessage request = new(HttpMethod.Get, uri) { Version = UseVersion };
                 using HttpResponseMessage response = await client.SendAsync(TestAsync, request, completionOption);
+                string responseContent = await response.Content.ReadAsStringAsync();
+
+                if (responseContentType == ResponseContentType.ContentLength)
+                {
+                    Assert.NotNull(response.Content.Headers.ContentLength);
+                }
+                else if (responseContentType == ResponseContentType.TransferEncodingChunked)
+                {
+                    Assert.NotNull(response.Headers.TransferEncodingChunked);
+                }
+                else
+                {
+                    // Empty
+                    Assert.Empty(responseContent);
+                }
 
                 Measurement<double> m = Assert.Single(recorder.GetMeasurements());
                 VerifyRequestDuration(m, uri, UseVersion, 200); ;
@@ -790,7 +805,7 @@ await Assert.ThrowsAsync<HttpRequestException>(async () =>
                         using HttpResponseMessage response = await SendAsync(client, request);
                     });
                 }
-                
+
                 Measurement<double> m = Assert.Single(recorder.GetMeasurements());
                 VerifyRequestDuration(m, uri, UseVersion, 200);
                 Assert.Equal("before!", m.Tags.ToArray().Single(t => t.Key == "before").Value);