chore(w3c): update tracestate tag regex (backport #4827) (#4836)

## Description
Right now _dd.propagation_error (and any internal tag that starts with
p) will get set as a tracestate tag. With this change, we make sure to
only add _dd.p.* tags.

## Checklist
- [ ] Followed the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
when writing a release note.
- [ ] Add additional sections for `feat` and `fix` pull requests.
- [ ] [Library
documentation](https://github.com/DataDog/dd-trace-py/tree/1.x/docs)
and/or [Datadog's documentation
site](https://github.com/DataDog/documentation/) is updated. Link to doc
PR in description.

<!-- Copy and paste the relevant snippet based on the type of pull
request -->

<!-- START feat -->

## Motivation
<!-- Expand on why the change is required, include relevant context for
reviewers -->

## Design 
<!-- Include benefits from the change as well as possible drawbacks and
trade-offs -->

## Testing strategy
<!-- Describe the automated tests and/or the steps for manual testing.

<!-- END feat -->

<!-- START fix -->

## Relevant issue(s)
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keywords for links to automate closing the issues once the pull request
is merged. -->

## Testing strategy
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performed. -->

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## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] Avoid breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is
included in the code or PR.
- [ ] Release note has been added and follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines),
or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.

Co-authored-by: Munir Abdinur <[email protected]>
Co-authored-by: Yun Kim <[email protected]>

Author: 3 people

ddtrace/internal/utils/http.py

@@ -171,7 +171,7 @@ def w3c_get_dd_list_member(context):
     for k, v in context._meta.items():
         if (
             isinstance(k, six.string_types)
-            and k.startswith("_dd.p")
+            and k.startswith("_dd.p.")
             # we've already added sampling decision and user id
             and k not in [SAMPLING_DECISION_TRACE_TAG_KEY, USER_ID_KEY]
         ):

tests/tracer/test_utils.py

@@ -418,6 +418,19 @@ def _():
             ),
             ["s:2", "o:synthetics", "t.unk:-4", "t.unknown:baz64"],
         ),
+        (
+            Context(
+                trace_id=1234,
+                sampling_priority=2,
+                dd_origin="synthetics",
+                meta={
+                    # we should not propagate _dd.propagation_error, since it does not start with _dd.p.
+                    "_dd.propagation_error": "-4",
+                    "_dd.p.unknown": "baz64",
+                },
+            ),
+            ["s:2", "o:synthetics", "t.unknown:baz64"],
+        ),
         (
             Context(
                 trace_id=1234,
@@ -473,6 +486,7 @@ def _():
     ],
     ids=[
         "basic",
+        "does_not_add_propagation_error",
         "does_not_add_non_prefixed_tags",
         "does_not_add_more_than_256_char",
         "char_replacement",