add v1 tracer interface so we can break out tracing (#523)

src/Ocelot/Logging/ITracer.cs

@@ -0,0 +1,19 @@
+namespace Ocelot.Logging
+{
+    using System;
+    using System.Net.Http;
+    using System.Threading;
+    using System.Threading.Tasks;
+    using Microsoft.AspNetCore.Http;
+
+    public interface ITracer
+    {
+        void Event(HttpContext httpContext, string @event);
+
+        Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request,
+            CancellationToken cancellationToken,
+            Action<string> addTraceIdToRepo,
+            Func<HttpRequestMessage, CancellationToken, Task<HttpResponseMessage>> baseSendAsync);
+    }
+}