fix(internal): writer recreate maintain extra headers [backport 2.7] (#…

…9429)

Backport 9e637e7 from #9424 to 2.7.

internal: this fixes an issue where a writer recreate call would make it
drop extra headers passed to the ``__init__``.

## Checklist

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

## Reviewer Checklist

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [x] 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)

Co-authored-by: Federico Mon <[email protected]>

ddtrace/internal/writer/writer.py

@@ -539,6 +539,7 @@ def recreate(self):
             dogstatsd=self.dogstatsd,
             sync_mode=self._sync_mode,
             api_version=self._api_version,
+            headers=self._headers,
         )
 
     @property

tests/tracer/test_writer.py

@@ -706,6 +706,16 @@ def test_writer_recreate_api_version(init_api_version, api_version, endpoint, en
     assert isinstance(writer._encoder, encoder_cls)
 
 
+def test_writer_recreate_keeps_headers():
+    writer = AgentWriter("http://dne:1234", headers={"Datadog-Client-Computed-Stats": "yes"})
+    assert "Datadog-Client-Computed-Stats" in writer._headers
+    assert writer._headers["Datadog-Client-Computed-Stats"] == "yes"
+
+    writer = writer.recreate()
+    assert "Datadog-Client-Computed-Stats" in writer._headers
+    assert writer._headers["Datadog-Client-Computed-Stats"] == "yes"
+
+
 @pytest.mark.parametrize(
     "sys_platform, api_version, ddtrace_api_version, priority_sampling, raises_error, expected",
     [