chore(context): update_tags only adds tags with new keys (backport #4061) (#4074)

This is an automatic backport of pull request #4061 done by [Mergify](https://mergify.com).


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Author: mergify[bot]

ddtrace/context.py

@@ -70,8 +70,10 @@ def _with_span(self, span):
     def _update_tags(self, span):
         # type: (Span) -> None
         with self._lock:
-            span._meta.update(self._meta)
-            span._metrics.update(self._metrics)
+            for tag in self._meta:
+                span._meta.setdefault(tag, self._meta[tag])
+            for metric in self._metrics:
+                span._metrics.setdefault(metric, self._metrics[metric])
 
     @property
     def sampling_priority(self):

tests/integration/test_integration_snapshots.py

@@ -5,8 +5,11 @@
 import pytest
 
 from ddtrace import Tracer
+from ddtrace.constants import AUTO_KEEP
 from ddtrace.constants import MANUAL_DROP_KEY
 from ddtrace.constants import MANUAL_KEEP_KEY
+from ddtrace.constants import SAMPLING_PRIORITY_KEY
+from ddtrace.constants import USER_KEEP
 from ddtrace.internal.writer import AgentWriter
 from ddtrace.sampler import DatadogSampler
 from ddtrace.sampler import RateSampler
@@ -273,3 +276,13 @@ def test_trace_with_wrong_metrics_types_not_sent(metrics):
                 with tracer.trace("child") as child:
                     child._metrics = metrics
         log.exception.assert_called_once_with("error closing trace")
+
+
+@snapshot()
+def test_tracetagsprocessor_only_adds_new_tags():
+    tracer = Tracer()
+    with tracer.trace(name="web.request") as span:
+        span.context.sampling_priority = AUTO_KEEP
+        span.set_metric(SAMPLING_PRIORITY_KEY, USER_KEEP)
+
+    tracer.shutdown()

tests/snapshots/tests.integration.test_integration_snapshots.test_tracetagsprocessor_only_adds_new_tags.json

@@ -0,0 +1,22 @@
+[[
+  {
+    "name": "web.request",
+    "service": null,
+    "resource": "web.request",
+    "trace_id": 0,
+    "span_id": 1,
+    "parent_id": 0,
+    "meta": {
+      "_dd.p.dm": "-0",
+      "runtime-id": "334311a6f22143ff855bbf68dbabac50"
+    },
+    "metrics": {
+      "_dd.agent_psr": 1.0,
+      "_dd.top_level": 1,
+      "_dd.tracer_kr": 1.0,
+      "_sampling_priority_v1": 2,
+      "system.pid": 15332
+    },
+    "duration": 34000,
+    "start": 1660010337590045000
+  }]]