fix(tracing): do not incorrectly match on None with ?* [backport 2.21] (#13279)

Backport ea64efc from #13100 to 2.21.

Fixes #12775

## Checklist
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- 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
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- Title is accurate
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Co-authored-by: Zachary Groves <[email protected]>
Co-authored-by: Brett Langdon <[email protected]>

Author: 3 people

ddtrace/_trace/sampling_rule.py

@@ -154,26 +154,25 @@ def check_tags(self, meta, metrics):
         tag_match = False
         for tag_key in self._tag_value_matchers.keys():
             value = meta.get(tag_key)
-            tag_match = self._tag_value_matchers[tag_key].match(str(value))
-            # If the value doesn't match in meta, check the metrics
-            if tag_match is False:
+            # it's because we're not checking metrics first before continuing
+            if value is None:
                 value = metrics.get(tag_key)
+                if value is None:
+                    continue
                 # Floats: Matching floating point values with a non-zero decimal part is not supported.
                 # For floating point values with a non-zero decimal part, any all * pattern always returns true.
                 # Other patterns always return false.
                 if isinstance(value, float):
                     if not value.is_integer():
-                        if self._tag_value_matchers[tag_key].pattern == "*":
+                        if all(c == "*" for c in self._tag_value_matchers[tag_key].pattern):
                             tag_match = True
+                            continue
                         else:
                             return False
-                        continue
                     else:
                         value = int(value)
 
-                tag_match = self._tag_value_matchers[tag_key].match(str(value))
-            else:
-                continue
+            tag_match = self._tag_value_matchers[tag_key].match(str(value))
             # if we don't match with all specified tags for a rule, it's not a match
             if tag_match is False:
                 return False

releasenotes/notes/fix_incorrect_sampler_match_on_none-c6e04cc4dfdf9007.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    fix(tracer):Fixed a bug in the sampling rule matcher where the pattern ``?*`` was not being matched correctly 
+    for ``DD_TRACE_SAMPLING_RULES`` tags, due to it matching on spans with no tag matching the specified key.

tests/tracer/test_sampler.py

@@ -932,6 +932,16 @@ def sample(self, span):
             0,
             None,
         ),
+        (  # Regression test for https://github.com/DataDog/dd-trace-py/issues/12775
+            # We should not match None values with ?* patterns.
+            DatadogSampler(
+                rules=[SamplingRule(sample_rate=0, name="?*", resource="?*", service="?*", tags={"key": "?*"})],
+            ),
+            AUTO_KEEP,
+            SamplingMechanism.DEFAULT,
+            0,
+            None,
+        ),
     ],
 )
 def test_datadog_sampler_sample_rules(sampler, sampling_priority, sampling_mechanism, rule, limit, dummy_tracer):