CP-52744: Update trace_context when starting a context span

Refresh the trace_context with the correct traceparent when creating a
span with `start_tracing_helper` in `context.ml`.

This ensures the tracing of the context has the correct parent.

ocaml/libs/tracing/tracing.ml

@@ -418,6 +418,12 @@ module Span = struct
         {span with status= {status_code; _description}}
     | _ ->
         span
+
+  let with_trace_context span trace_context =
+    let span_context =
+      span |> get_context |> SpanContext.with_trace_context trace_context
+    in
+    {span with context= span_context}
 end
 
 module TraceMap = Map.Make (Trace_id)

ocaml/libs/tracing/tracing.mli

@@ -146,6 +146,8 @@ module Span : sig
   val get_end_time : t -> float option
 
   val get_attributes : t -> (string * string) list
+
+  val with_trace_context : t -> TraceContext.t -> t
 end
 
 module TraceMap : module type of Map.Make (Trace_id)

ocaml/xapi/context.ml

@@ -337,7 +337,20 @@ let start_tracing_helper ?(span_attributes = []) parent_fn task_name =
       ~parent ()
   with
   | Ok x ->
-      x
+      Option.map
+        (fun span ->
+          let traceparent =
+            span |> Span.get_context |> SpanContext.to_traceparent
+          in
+          let trace_context =
+            span
+            |> Span.get_context
+            |> SpanContext.context_of_span_context
+            |> TraceContext.with_traceparent (Some traceparent)
+          in
+          Span.with_trace_context span trace_context
+        )
+        x
   | Error e ->
       R.warn "Failed to start tracing: %s" (Printexc.to_string e) ;
       None