fix(tracing_utils): resolves RuntimeError dictionary changed size during iteration [backport 1.20] (#7808)

Backport 215f2bf from #7427 to 1.20.

We've been seeing this infrequently in a FastAPI app and decided to dig
in.

Looks like `context._meta` is changing size during iteration.

Saw a similar issue
[here](#1745) so implementing
a similar fix.

---

`1.x` PR: #7439

## Checklist

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- [x] Testing strategy is described if automated tests are not included
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- [x] Risk is outlined (performance impact, potential for breakage,
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- [x] Change is maintainable (easy to change, telemetry, documentation).
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
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[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
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Co-authored-by: Gil Aviv <[email protected]>

Author: github-actions[bot]

ddtrace/internal/utils/__init__.py

@@ -77,7 +77,8 @@ def set_argument_value(
 def _get_metas_to_propagate(context):
     # type: (Any) -> List[Tuple[str, str]]
     metas_to_propagate = []
-    for k, v in context._meta.items():
+    # copying context._meta.items() to avoid RuntimeError: dictionary changed size during iteration
+    for k, v in list(context._meta.items()):
         if isinstance(k, six.string_types) and k.startswith("_dd.p."):
             metas_to_propagate.append((k, v))
     return metas_to_propagate

releasenotes/notes/fix-runtimeerror-on-get-metas-to-propagate-25f9c44ac4137fb9.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    tracing: This fix resolves an issue where concurrent mutations to the ``context._meta`` dict
+    caused `RuntimeError: dictionary changed size during iteration`.