Add metrics exporter modes (#2353)

src/OpenTelemetry.Exporter.Prometheus/PrometheusExporter.cs

@@ -24,6 +24,7 @@ namespace OpenTelemetry.Exporter
     /// Exporter of OpenTelemetry metrics to Prometheus.
     /// </summary>
     [AggregationTemporality(AggregationTemporality.Cumulative)]
+    [ExportModes(ExportModes.Pull)]
     public class PrometheusExporter : BaseExporter<Metric>
     {
         internal readonly PrometheusExporterOptions Options;

src/OpenTelemetry/Metrics/BaseExportingMetricReader.cs

@@ -23,19 +23,31 @@ public class BaseExportingMetricReader : MetricReader
         protected readonly BaseExporter<Metric> exporter;
         protected bool disposed;
 
+        private readonly ExportModes supportedExportModes = ExportModes.Push | ExportModes.Pull;
+
         public BaseExportingMetricReader(BaseExporter<Metric> exporter)
         {
             this.exporter = exporter ?? throw new ArgumentNullException(nameof(exporter));
 
-            var attributes = exporter.GetType().GetCustomAttributes(typeof(AggregationTemporalityAttribute), true);
+            var exportorType = exporter.GetType();
+            var attributes = exportorType.GetCustomAttributes(typeof(AggregationTemporalityAttribute), true);
+            if (attributes.Length > 0)
+            {
+                var attr = (AggregationTemporalityAttribute)attributes[attributes.Length - 1];
+                this.PreferredAggregationTemporality = attr.Preferred;
+                this.SupportedAggregationTemporality = attr.Supported;
+            }
+
+            attributes = exportorType.GetCustomAttributes(typeof(ExportModesAttribute), true);
             if (attributes.Length > 0)
             {
-                AggregationTemporalityAttribute aggregationTemporality = (AggregationTemporalityAttribute)attributes[attributes.Length - 1];
-                this.PreferredAggregationTemporality = aggregationTemporality.Preferred;
-                this.SupportedAggregationTemporality = aggregationTemporality.Supported;
+                var attr = (ExportModesAttribute)attributes[attributes.Length - 1];
+                this.supportedExportModes = attr.Supported;
             }
         }
 
+        protected ExportModes SupportedExportModes => this.supportedExportModes;
+
         public override void OnCollect(Batch<Metric> metrics)
         {
             this.exporter.Export(metrics);

src/OpenTelemetry/Metrics/ExportModes.cs

@@ -0,0 +1,47 @@
+// <copyright file="ExportModes.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>
+
+using System;
+
+namespace OpenTelemetry.Metrics
+{
+    [Flags]
+    public enum ExportModes : byte
+    {
+        /*
+        0 0 0 0 0 0 0 0
+        | | | | | | | |
+        | | | | | | | +--- Push
+        | | | | | | +----- Pull
+        | | | | | +------- (reserved)
+        | | | | +--------- (reserved)
+        | | | +----------- (reserved)
+        | | +------------- (reserved)
+        | +--------------- (reserved)
+        +----------------- (reserved)
+        */
+
+        /// <summary>
+        /// Push.
+        /// </summary>
+        Push = 0b1,
+
+        /// <summary>
+        /// Pull.
+        /// </summary>
+        Pull = 0b10,
+    }
+}

src/OpenTelemetry/Metrics/ExportModesAttribute.cs

@@ -0,0 +1,33 @@
+// <copyright file="ExportModesAttribute.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>
+
+using System;
+
+namespace OpenTelemetry.Metrics
+{
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
+    public sealed class ExportModesAttribute : Attribute
+    {
+        private ExportModes supportedExportModes;
+
+        public ExportModesAttribute(ExportModes supported)
+        {
+            this.supportedExportModes = supported;
+        }
+
+        public ExportModes Supported => this.supportedExportModes;
+    }
+}

src/OpenTelemetry/Metrics/PeriodicExportingMetricReader.cs

@@ -22,20 +22,31 @@ namespace OpenTelemetry.Metrics
 {
     public class PeriodicExportingMetricReader : BaseExportingMetricReader
     {
+        internal const int DefaultExportIntervalMilliseconds = 60000;
+        internal const int DefaultExportTimeoutMilliseconds = 30000;
+
         private readonly Task exportTask;
         private readonly CancellationTokenSource token;
 
-        public PeriodicExportingMetricReader(BaseExporter<Metric> exporter, int exportIntervalMs)
+        public PeriodicExportingMetricReader(
+            BaseExporter<Metric> exporter,
+            int exportIntervalMilliseconds = DefaultExportIntervalMilliseconds,
+            int exportTimeoutMilliseconds = DefaultExportTimeoutMilliseconds)
             : base(exporter)
         {
+            if ((this.SupportedExportModes & ExportModes.Push) != ExportModes.Push)
+            {
+                throw new InvalidOperationException("The exporter does not support push mode.");
+            }
+
             this.token = new CancellationTokenSource();
 
             // TODO: Use dedicated thread.
             this.exportTask = new Task(() =>
             {
                 while (!this.token.IsCancellationRequested)
                 {
-                    Task.Delay(exportIntervalMs).Wait();
+                    Task.Delay(exportIntervalMilliseconds).Wait();
                     this.Collect();
                 }
             });