Respect parent_sampled decision in propagation_context sentry-trace header (#4356)

Since we don't automatically have unsampled spans running, this caused a
change in behavior when an upstream sampling decision needs to be
propagated further downstream.

### Explanation of problem
When an incoming trace has `sampled` set to 0 (`trace_id-span_id-0`),
in the past we would propagate this since we would have an active
span/transaction running but just not sampled, so downstream would also
receive `trace_id-span_id-0` from that active span.
Now, we actually don't have an active span since we don't sample (just
how otel works), so instead of sending the `trace_id-span_id-0` as
before, we would have sent `trace_id-other_span_id` from the
`propagation_context` instead.
This would cause the downstream service to not receive the `-0` flag and
would thus sample independently, which is a regression.

Author: sl0thentr0py

sentry_sdk/scope.py

@@ -505,11 +505,7 @@ def get_traceparent(self, *args, **kwargs):
 
         # If this scope has a propagation context, return traceparent from there
         if self._propagation_context is not None:
-            traceparent = "%s-%s" % (
-                self._propagation_context.trace_id,
-                self._propagation_context.span_id,
-            )
-            return traceparent
+            return self._propagation_context.to_traceparent()
 
         # Fall back to isolation scope's traceparent. It always has one
         return self.get_isolation_scope().get_traceparent()

sentry_sdk/tracing_utils.py

@@ -432,6 +432,21 @@ def span_id(self, value):
         # type: (str) -> None
         self._span_id = value
 
+    def to_traceparent(self):
+        # type: () -> str
+        if self.parent_sampled is True:
+            sampled = "1"
+        elif self.parent_sampled is False:
+            sampled = "0"
+        else:
+            sampled = None
+
+        traceparent = "%s-%s" % (self.trace_id, self.span_id)
+        if sampled is not None:
+            traceparent += "-%s" % (sampled,)
+
+        return traceparent
+
     def update(self, other_dict):
         # type: (Dict[str, Any]) -> None
         """