Add EventListener for SelfDiagnostics (#2259)

* Add EventListener for SelfDiagnostics

* Match with EventSource name's prefix

BASE/src/Microsoft.ApplicationInsights/Extensibility/Implementation/Tracing/SelfDiagnosticsInternals/SelfDiagnosticsEventListener.cs

@@ -0,0 +1,93 @@
+namespace Microsoft.ApplicationInsights.Extensibility.Implementation.Tracing.SelfDiagnosticsInternals
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Collections.ObjectModel;
+    using System.Diagnostics.Tracing;
+    using System.IO;
+    using System.Text;
+    using System.Threading;
+
+    /// <summary>
+    /// SelfDiagnosticsEventListener class enables the events from OpenTelemetry event sources
+    /// and write the events to a local file in a circular way.
+    /// </summary>
+    internal class SelfDiagnosticsEventListener : EventListener
+    {
+        private const string EventSourceNamePrefix = "Microsoft-ApplicationInsights-";
+        private readonly object lockObj = new object();
+        private readonly EventLevel logLevel;
+
+        // private readonly SelfDiagnosticsConfigRefresher configRefresher;
+        private readonly List<EventSource> eventSourcesBeforeConstructor = new List<EventSource>();
+
+        public SelfDiagnosticsEventListener(EventLevel logLevel/*, SelfDiagnosticsConfigRefresher configRefresher*/)
+        {
+            this.logLevel = logLevel;
+
+            // this.configRefresher = configRefresher ?? throw new ArgumentNullException(nameof(configRefresher));
+
+            List<EventSource> eventSources;
+            lock (this.lockObj)
+            {
+                eventSources = this.eventSourcesBeforeConstructor;
+                this.eventSourcesBeforeConstructor = null;
+            }
+
+            foreach (var eventSource in eventSources)
+            {
+#if NET452
+                this.EnableEvents(eventSource, this.logLevel, (EventKeywords)(-1));
+#else
+                this.EnableEvents(eventSource, this.logLevel, EventKeywords.All);
+#endif
+            }
+        }
+
+        internal void WriteEvent(string eventMessage, ReadOnlyCollection<object> payload)
+        {
+            // TODO
+        }
+
+        protected override void OnEventSourceCreated(EventSource eventSource)
+        {
+            if (eventSource.Name.StartsWith(EventSourceNamePrefix, StringComparison.Ordinal))
+            {
+                // If there are EventSource classes already initialized as of now, this method would be called from
+                // the base class constructor before the first line of code in SelfDiagnosticsEventListener constructor.
+                // In this case logLevel is always its default value, "LogAlways".
+                // Thus we should save the event source and enable them later, when code runs in constructor.
+                if (this.eventSourcesBeforeConstructor != null)
+                {
+                    lock (this.lockObj)
+                    {
+                        if (this.eventSourcesBeforeConstructor != null)
+                        {
+                            this.eventSourcesBeforeConstructor.Add(eventSource);
+                            return;
+                        }
+                    }
+                }
+
+#if NET452
+                this.EnableEvents(eventSource, this.logLevel, (EventKeywords)(-1));
+#else
+                this.EnableEvents(eventSource, this.logLevel, EventKeywords.All);
+#endif
+            }
+
+            base.OnEventSourceCreated(eventSource);
+        }
+
+        /// <summary>
+        /// This method records the events from event sources to a local file, which is provided as a stream object by
+        /// SelfDiagnosticsConfigRefresher class. The file size is bound to a upper limit. Once the write position
+        /// reaches the end, it will be reset to the beginning of the file.
+        /// </summary>
+        /// <param name="eventData">Data of the EventSource event.</param>
+        protected override void OnEventWritten(EventWrittenEventArgs eventData)
+        {
+            this.WriteEvent(eventData.Message, eventData.Payload);
+        }
+    }
+}