add comment for circular dependency

packages/core/src/utils/spanUtils.ts

@@ -60,6 +60,11 @@ function ensureTimestampInSeconds(timestamp: number): number {
 /**
  * Convert a span to a JSON representation.
  * Note that all fields returned here are optional and need to be guarded against.
+ *
+ * Note: Because of this, we currently have a circular type dependency (which we opted out of in package.json).
+ * This is not avoidable as we need `spanToJSON` in `spanUtils.ts`, which in turn is needed by `span.ts` for backwards compatibility.
+ * And `spanToJSON` needs the Span class from `span.ts` to check here.
+ * TODO v8: When we remove the deprecated stuff from `span.ts`, we can remove the circular dependency again.
  */
 export function spanToJSON(span: Span): Partial<SpanJSON> {
   if (spanIsSpanClass(span)) {