@@ -14,8 +14,16 @@
 // limitations under the License.
 // </copyright>
 
+using System;
+using System.Collections.Generic;
+
 namespace OpenTelemetry.Metrics
 {
+    // TODO: Clarify if spec allows adding anything from context
+    // or only Baggage entries. If the later, we just need to accept the
+    // Baggage key names only, and SDK can internally do Baggage.GetBaggage(name)
+    public delegate ReadOnlySpan<KeyValuePair<string, object>> AdditionalTagsCallback();
+
     // TODO: can be optimized like MetricType
     public enum Aggregation
     {
// <copyright file="MetricStreamConfiguration.cs" company="OpenTelemetry Authors">
// Copyright The OpenTelemetry Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// </copyright>

namespace OpenTelemetry.Metrics
{
    // TODO: can be optimized like MetricType
    public enum Aggregation
    {
#pragma warning disable SA1602 // Enumeration items should be documented
        Default,
        Drop,
        Sum,
        LastValue,
        Histogram,
#pragma warning restore SA1602 // Enumeration items should be documented
    }

    public class MetricStreamConfiguration
    {
        public static readonly MetricStreamConfiguration Drop = new DropConfiguration();

        public string Name { get; set; }

        public string Description { get; set; }

        public string[] TagKeys { get; set; }

        public virtual Aggregation Aggregation { get; set; }

        // TODO: MetricPoints caps can be configured here

        private sealed class DropConfiguration : MetricStreamConfiguration
        {
            public override Aggregation Aggregation
            {
                get => Aggregation.Drop;
                set { }
            }
        }
    }
}

@reyang need some input from you here.