[WIP] In-process propagation with a simple ITracer.ActiveSpan property

src/OpenTracing/ISpanBuilder.cs

@@ -36,6 +36,11 @@ public interface ISpanBuilder
         /// e.g., for a <see cref="References.ChildOf"/> referenceType, the referencedContext is the parent.</param>
         ISpanBuilder AddReference(string referenceType, ISpanContext referencedContext);
 
+        /// <summary>
+        /// When used, the newly created span will NOT inherit <see cref="ITracer.ActiveSpan"/> as a parent.
+        /// </summary>
+        ISpanBuilder IgnoreActiveSpan();
+
         /// <summary>
         /// Adds a tag to the span.
         /// </summary>

src/OpenTracing/ITracer.cs

@@ -7,6 +7,13 @@ namespace OpenTracing
     /// </summary>
     public interface ITracer
     {
+        /// <summary>
+        /// <para>Returns the currently active span. It is used for intra-process propagation and follows the async execution flow.</para>
+        /// <para>Any newly started span will inherit this span as a parent unless <see cref="ISpanBuilder.IgnoreActiveSpan"/>
+        /// is called before the span is started.</para>
+        /// </summary>
+        ISpan ActiveSpan { get; }
+
         /// <summary>
         /// Returns a new <see cref="ISpanBuilder" /> for a span with the given <paramref name="operationName" />.
         /// </summary>

src/OpenTracing/NullTracer/NullSpanBuilder.cs

@@ -35,6 +35,11 @@ public ISpanBuilder AddReference(string referenceType, ISpanContext referencedCo
             return this;
         }
 
+        public ISpanBuilder IgnoreActiveSpan()
+        {
+            return this;
+        }
+
         public ISpanBuilder WithStartTimestamp(DateTime startTimestamp)
         {
             return this;

src/OpenTracing/NullTracer/NullTracer.cs

@@ -10,6 +10,8 @@ private NullTracer()
         {
         }
 
+        public ISpan ActiveSpan => NullSpan.Instance;
+
         public ISpanBuilder BuildSpan(string operationName)
         {
             return NullSpanBuilder.Instance;