stats: mark InPayload.Data and OutPayload.Data for deletion (experimental) (#7121)

diff --git a/stats/stats.go b/stats/stats.go
index 4ab70e2..fdb0bd6 100644
--- a/stats/stats.go
+++ b/stats/stats.go
@@ -73,9 +73,12 @@
 type InPayload struct {
 	// Client is true if this InPayload is from client side.
 	Client bool
-	// Payload is the payload with original type.
+	// Payload is the payload with original type.  This may be modified after
+	// the call to HandleRPC which provides the InPayload returns and must be
+	// copied if needed later.
 	Payload any
 	// Data is the serialized message payload.
+	// Deprecated: Data will be removed in the next release.
 	Data []byte
 
 	// Length is the size of the uncompressed payload data. Does not include any
@@ -143,9 +146,12 @@
 type OutPayload struct {
 	// Client is true if this OutPayload is from client side.
 	Client bool
-	// Payload is the payload with original type.
+	// Payload is the payload with original type.  This may be modified after
+	// the call to HandleRPC which provides the OutPayload returns and must be
+	// copied if needed later.
 	Payload any
 	// Data is the serialized message payload.
+	// Deprecated: Data will be removed in the next release.
 	Data []byte
 	// Length is the size of the uncompressed payload data. Does not include any
 	// framing (gRPC or HTTP/2).