fix(tracing): preserve generator return values in @tracer.wrap() decorator (#13784)

The @tracer.wrap() decorator was not properly preserving return values
from generator functions. When a wrapped generator function returned a
value, the StopIteration exception's value attribute was None instead of
the actual returned value.

This fix ensures that generator return values are properly captured and
propagated through the StopIteration exception by storing the return
value from 'yield from' and returning it after the tracing context
exits.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [ ] Reviewer has checked that all the criteria below are met
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

(cherry picked from commit 4b87ee8)

Author: brettlangdon

ddtrace/_trace/tracer.py

@@ -775,7 +775,8 @@ def func_wrapper(*args, **kwargs):
                 )
 
             with self.trace(span_name, service=service, resource=resource, span_type=span_type):
-                yield from f(*args, **kwargs)
+                return_value = yield from f(*args, **kwargs)
+                return return_value
 
         return func_wrapper
 

releasenotes/notes/fix-tracer-wrap-generator-return-value-fbaec09a9670e06b.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    tracing: This fix resolves an issue where the ``@tracer.wrap()`` decorator failed to preserve return values from generator functions, causing ``StopIteration.value`` to be ``None`` instead of the actual returned value.

tests/tracer/test_tracer.py

@@ -355,6 +355,28 @@ def wrapper():
             "visible to the caller"
         )
 
+    def test_tracer_wrap_generator_with_return_value(self):
+        @self.tracer.wrap()
+        def iter_signals():
+            for i in range(10):
+                yield i
+            return 10
+
+        with self.trace("root") as span:
+            signals = iter_signals()
+            while True:
+                try:
+                    # DEV: We don't need the return value
+                    next(signals)
+                except StopIteration as e:
+                    assert e.value == 10
+                    span.set_metric("num_signals", e.value)
+                    break
+
+        self.assert_span_count(2)
+        root_span = self.get_root_span()
+        root_span.assert_matches(name="root", metrics={"num_signals": 10})
+
     def test_tracer_disabled(self):
         self.tracer.enabled = True
         with self.trace("foo") as s: