Add trace propagation

Author: cleptric

develop-docs/sdk/telemetry/spans/span-properties.mdx

@@ -0,0 +1,26 @@
+---
+title: Span Trace Propagation
+---
+
+# Span Trace Propagation
+
+## Continue an incoming trace
+
+To continue a trace from an upstream service, the SDK must expose a method to extract the traceparent and baggage information and apply these to the applicable scope.
+
+```js
+scope.setPropagationContext({
+    traceparent: request.headers.SENTRY_TRACE,
+    baggage: request.headers.SENTRY_BAGGAGE,
+})
+```
+Newly created root spans should now contain these properties, such as `trace_id` and `parent_span_id`.
+
+## Continue an outgoing trace
+
+To propagate a trace to a downstream service, the SDK must expose methods to fetch the required information to allow the next service to continue the trace.
+
+```js
+traceparent = scope.getTraceparent()
+baggage = scope.getBaggage()
+```