Adds support for optionally recording the policy (and any composite policy) associated with an inclusive tail processor sampling decision.

Resolves !35180.

- This functionality lives behind a feature flag that is disabled by default
- The original issue described a solution where we might attach the attribute solely to the root span. I'm not sure I agree with the commenter that we can rely on this (e.g. we might decide to sample halfway through a long-running trace) so I have attached the attributes to all present scope spans. This feels like a decent trade off between complexity + network cost, as finding the highest non-root parent would require multiple passes of the spans and keeping all span ids in a set

- Added automated tests to verify enabling the flag both records the expected decision while not impacting existing logic
- Built a custom version and ran it in our preprod environment to ensure it was stable over a 1h period (still evaluating, will update PR with any further observations)

Does this require a CHANGELOG entry?

Author: djluck

processor/tailsamplingprocessor/composite_helper.go

@@ -4,6 +4,7 @@
 package tailsamplingprocessor // import "github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor"
 
 import (
+	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor/internal/telemetry"
 	"go.opentelemetry.io/collector/component"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor/internal/telemetry"

processor/tailsamplingprocessor/factory.go

@@ -7,6 +7,7 @@ package tailsamplingprocessor // import "github.com/open-telemetry/opentelemetry
 
 import (
 	"context"
+	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor/internal/telemetry"
 	"time"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/processor/tailsamplingprocessor/internal/telemetry"

processor/tailsamplingprocessor/processor_decisions_test.go

@@ -5,6 +5,7 @@ package tailsamplingprocessor
 
 import (
 	"context"
+	"github.com/stretchr/testify/assert"
 	"testing"
 	"time"