chore: modify graphql span event traceback (#12324)

Modify graphql span event `error.stack` formatting for consistency with
`exception.stacktrace` as here:
#12185

If these are not exactly the same, this might lead to issues with error
tracking.

## Checklist
- [x] PR author has checked that all the criteria below are met
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- 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
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] 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
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- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
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ddtrace/contrib/internal/graphql/patch.py

@@ -1,3 +1,4 @@
+from io import StringIO
 import os
 import re
 import sys
@@ -339,13 +340,13 @@ def _set_span_errors(errors: List[GraphQLError], span: Span) -> None:
         }
 
         if error.__traceback__:
-            stacktrace = "\n".join(
-                traceback.format_exception(
-                    type(error), error, error.__traceback__, limit=config._span_traceback_max_size
-                )
-            )
-            attributes["stacktrace"] = stacktrace
-            span.set_tag_str(ERROR_STACK, stacktrace)
+            exc_type, exc_val, exc_tb = type(error), error, error.__traceback__
+            buff = StringIO()
+            traceback.print_exception(exc_type, exc_val, exc_tb, file=buff, limit=config._span_traceback_max_size)
+            tb = buff.getvalue()
+
+            attributes["stacktrace"] = tb
+            span.set_tag_str(ERROR_STACK, tb)
 
         if error.path is not None:
             path = ",".join([str(path_obj) for path_obj in error.path])