feat(tracing): Add Span.SetDynamicSamplingContext method (#539)

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Misc
 
 - Add SentryTraceHeader and SentryBaggageHeader constants ([#538](https://github.com/getsentry/sentry-go/pull/538/))
+- Add `Span.SetDynamicSamplingContext` method ([#539](https://github.com/getsentry/sentry-go/pull/539/))
 - Add DSN getters ([#540](https://github.com/getsentry/sentry-go/pull/540))
 
 ## 0.17.0

tracing.go

@@ -240,6 +240,14 @@ func (s *Span) ToBaggage() string {
 	return s.dynamicSamplingContext.String()
 }
 
+// SetDynamicSamplingContext sets the given dynamic sampling context on the
+// current transaction.
+func (s *Span) SetDynamicSamplingContext(dsc DynamicSamplingContext) {
+	if s.isTransaction {
+		s.dynamicSamplingContext = dsc
+	}
+}
+
 // sentryTracePattern matches either
 //
 //	TRACE_ID - SPAN_ID

tracing_test.go

@@ -654,3 +654,38 @@ func TestDoesNotCrashWithEmptyContext(t *testing.T) {
 	tx.Sampled = SampledTrue
 	tx.Finish()
 }
+
+func TestSetDynamicSamplingContextWorksOnTransaction(t *testing.T) {
+	s := Span{
+		isTransaction:          true,
+		dynamicSamplingContext: DynamicSamplingContext{Frozen: false},
+	}
+	newDsc := DynamicSamplingContext{
+		Entries: map[string]string{"environment": "dev"},
+		Frozen:  true,
+	}
+
+	s.SetDynamicSamplingContext(newDsc)
+
+	if diff := cmp.Diff(newDsc, s.dynamicSamplingContext); diff != "" {
+		t.Errorf("DynamicSamplingContext mismatch (-want +got):\n%s", diff)
+	}
+}
+
+func TestSetDynamicSamplingContextDoesNothingOnSpan(t *testing.T) {
+	// SetDynamicSamplingContext should do nothing on non-transaction spans
+	s := Span{
+		isTransaction:          false,
+		dynamicSamplingContext: DynamicSamplingContext{},
+	}
+	newDsc := DynamicSamplingContext{
+		Entries: map[string]string{"environment": "dev"},
+		Frozen:  true,
+	}
+
+	s.SetDynamicSamplingContext(newDsc)
+
+	if diff := cmp.Diff(DynamicSamplingContext{}, s.dynamicSamplingContext); diff != "" {
+		t.Errorf("DynamicSamplingContext mismatch (-want +got):\n%s", diff)
+	}
+}