Fix build

Author: JamesNK

src/Shared/Metrics/MetricsConstants.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Generic;
+
 namespace Microsoft.AspNetCore.Http;
 
 internal static class MetricsConstants