Simplify continue_trace to reuse propagation_context values (#5158)

### Description

We are already parsing both `sentry-trace` and `baggage` headers and
filling in `sample_rand` while constructing the `propagation_context`.
We don't need the redundant logic in `Transaction.continue_from_headers`
anymore.

Author: sl0thentr0py

sentry_sdk/scope.py

@@ -1186,28 +1186,26 @@ def continue_trace(
         """
         self.generate_propagation_context(environ_or_headers)
 
-        # When we generate the propagation context, the sample_rand value is set
-        # if missing or invalid (we use the original value if it's valid).
-        # We want the transaction to use the same sample_rand value. Due to duplicated
-        # propagation logic in the transaction, we pass it in to avoid recomputing it
-        # in the transaction.
-        # TYPE SAFETY: self.generate_propagation_context() ensures that self._propagation_context
-        # is not None.
-        sample_rand = typing.cast(
-            PropagationContext, self._propagation_context
-        )._sample_rand()
-
-        transaction = Transaction.continue_from_headers(
-            normalize_incoming_data(environ_or_headers),
-            _sample_rand=sample_rand,
+        # generate_propagation_context ensures that the propagation_context is not None.
+        propagation_context = typing.cast(PropagationContext, self._propagation_context)
+
+        optional_kwargs = {}
+        if name:
+            optional_kwargs["name"] = name
+        if source:
+            optional_kwargs["source"] = source
+
+        return Transaction(
             op=op,
             origin=origin,
-            name=name,
-            source=source,
+            baggage=propagation_context.baggage,
+            parent_sampled=propagation_context.parent_sampled,
+            trace_id=propagation_context.trace_id,
+            parent_span_id=propagation_context.parent_span_id,
+            same_process_as_parent=False,
+            **optional_kwargs,
         )
 
-        return transaction
-
     def capture_event(self, event, hint=None, scope=None, **scope_kwargs):
         # type: (Event, Optional[Hint], Optional[Scope], Any) -> Optional[str]
         """