Add traceStateValue parameter to DistributedTracingData.TryDeserializ… (#2607)

We pass the 'traceparent' value in our asynchronous message
communication.
We follow the instructions at
https://www.elastic.co/guide/en/apm/get-started/7.10/distributed-tracing.html#_manual_distributed_tracing.
However, it is not possible to restore the 'tracestate' as it is for the
Go agent implementation.

In pair 'restart_external' TraceContinuationStrategy, it is not possible
to search the logs across services using the original traceid, as each
message handling generates anew traceid due to the absence of a
tracestate.

The PR suggests a possible solution.

Author: otemnov

src/Elastic.Apm/Api/DistributedTracingData.cs

@@ -33,16 +33,25 @@ internal DistributedTracingData(string traceId, string parentId, bool flagRecord
 		internal TraceState TraceState { get; }
 
 		/// <summary>
-		/// Serializes this instance to a string.
-		/// This method should be used at the caller side and the return value should be passed to the (possibly remote) callee
-		/// side.
-		/// <see cref="TryDeserializeFromString" /> should be used to deserialize the instance at the callee side.
+		/// Serializes this instance to a traceparent string.
+		/// This method should be used at the caller side in pairs with <see cref="SerializeTraceStateToString" />,
+		/// and the return value should be passed to the (possibly remote) callee side.
+		/// <see cref="Create" /> should be used to deserialize the instance at the callee side.
 		/// </summary>
 		/// <returns>
-		/// String containing the instance in serialized form.
+		/// String containing the traceparent data in serialized form.
 		/// </returns>
 		public string SerializeToString() => TraceContext.BuildTraceparent(this);
 
+		/// <summary>
+		/// Serializes this instance to a tracestate string.
+		/// This method should be used at the caller side in pairs with <see cref="SerializeToString" />,
+		/// and the return value should be passed to the (possibly remote) callee side.
+		/// <see cref="Create" /> should be used to deserialize the instance at the callee side.
+		/// </summary>
+		/// <returns>The string representation of the tracestate header, or null if there is no tracestate.</returns>
+		public string SerializeTraceStateToString() => TraceState?.ToTextHeader();
+
 		/// <summary>
 		/// Deserializes an instance from a string.
 		/// This method should be used at the callee side and the deserialized instance can be passed to
@@ -54,6 +63,19 @@ internal DistributedTracingData(string traceId, string parentId, bool flagRecord
 		/// </returns>
 		public static DistributedTracingData TryDeserializeFromString(string serialized) => TraceContext.TryExtractTracingData(serialized);
 
+		/// <summary>
+		/// Creates an instance from a treceparent and tracestate strings.
+		/// This method should be used at the callee side, and the created instance can be passed to
+		/// <see cref="ITracer.StartTransaction" />.
+		/// </summary>
+		/// <param name="traceParent">should be a return value from a call to <see cref="SerializeToString" />.</param>
+		/// <param name="traceState">should be a return value from a call to <see cref="SerializeTraceStateToString"/>.</param>
+		/// <returns>
+		/// Instance deserialized from <paramref name="traceParent" /> and <paramref name="traceState" /> .
+		/// </returns>
+		public static DistributedTracingData Create(string traceParent, string traceState) =>
+			TraceContext.TryExtractTracingData(traceParent, traceState);
+
 		public override string ToString() => new ToStringBuilder(nameof(DistributedTracingData))
 		{
 			{ nameof(TraceId), TraceId },