fix(traces): keep backend as safety net for trace stats when tracer claims client-computed-stats

The Datadog-Client-Computed-Stats header should only suppress extension-side
stats generation, not backend stats. When the extension is not computing stats
itself, always set _dd.compute_stats:1 so the backend remains a fallback.

Author: duncanista

bottlecap/src/lifecycle/invocation/processor.rs

@@ -2255,8 +2255,9 @@ mod tests {
             "client_computed_stats must be propagated to the aws.lambda span payload"
         );
 
-        // Verify _dd.compute_stats is "0" in the built payload tags: client_computed_stats=true
-        // means the tracer has already computed stats, so neither extension nor backend should.
+        // Verify _dd.compute_stats is "1" in the built payload tags: the backend is always
+        // the safety net when compute_trace_stats_on_extension is false, regardless of
+        // client_computed_stats.
         let send_data = payload.builder.build();
         let libdd_trace_utils::tracer_payload::TracerPayloadCollection::V07(payloads) =
             send_data.get_payloads()
@@ -2266,8 +2267,8 @@ mod tests {
         for p in payloads {
             assert_eq!(
                 p.tags.get(crate::tags::lambda::tags::COMPUTE_STATS_KEY),
-                Some(&"0".to_string()),
-                "_dd.compute_stats must be 0 when client_computed_stats is true"
+                Some(&"1".to_string()),
+                "_dd.compute_stats must be 1 (backend safety net) when compute_trace_stats_on_extension is false"
             );
         }
     }

bottlecap/src/traces/trace_processor.rs

@@ -351,14 +351,19 @@ impl TraceProcessor for ServerlessTraceProcessor {
             for tracer_payload in collection.iter_mut() {
                 tracer_payload.tags.extend(tags.clone());
                 // Tell the backend whether to compute stats:
-                // - "1" (compute on backend) if neither the tracer nor the extension is computing them
-                // - "0" (skip on backend) if the extension or the tracer has already computed them
-                let compute_stats = if !config.compute_trace_stats_on_extension
-                    && !header_tags.client_computed_stats
-                {
-                    "1"
-                } else {
+                // - "1" (compute on backend) if the extension is NOT computing stats
+                // - "0" (skip on backend) if the extension is computing stats
+                //
+                // We intentionally ignore client_computed_stats here to keep
+                // the backend as a safety net. If the tracer claims it computed
+                // stats but they don't actually reach the backend, setting this
+                // to "0" would cause stats to disappear entirely.
+                // This may lead to double-counting when the tracer also sends
+                // stats, but that is preferable to zero stats.
+                let compute_stats = if config.compute_trace_stats_on_extension {
                     "0"
+                } else {
+                    "1"
                 };
                 tracer_payload
                     .tags
@@ -1400,7 +1405,7 @@ mod tests {
     /// | Input: `compute_trace_stats_on_extension` | Input: `client_computed_stats` | Expected: `_dd.compute_stats` | Expected: Extension generates stats? |
     /// |-------------------------------------------|--------------------------------|-------------------------------|--------------------------------------|
     /// | `false`                                   | `false`                        | `"1"`                         | No                                   |
-    /// | `false`                                   | `true`                         | `"0"`                         | No                                   |
+    /// | `false`                                   | `true`                         | `"1"`                         | No                                   |
     /// | `true`                                    | `false`                        | `"0"`                         | Yes                                  |
     /// | `true`                                    | `true`                         | `"0"`                         | No                                   |
     #[allow(clippy::unwrap_used)]
@@ -1414,11 +1419,12 @@ mod tests {
         use libdd_trace_obfuscation::obfuscation_config::ObfuscationConfig;
         use tokio::sync::mpsc;
 
-        // "_dd.compute_stats" is "1" only when neither side computes stats (backend must do it).
-        let expected_tag = if !compute_trace_stats_on_extension && !client_computed_stats {
-            "1"
-        } else {
+        // "_dd.compute_stats" is "1" when the extension is NOT computing stats (backend must do it).
+        // client_computed_stats does NOT affect this tag — the backend is the safety net.
+        let expected_tag = if compute_trace_stats_on_extension {
             "0"
+        } else {
+            "1"
         };
         // The extension generates stats only when it is configured to do so and the tracer hasn't.
         let expect_stats = compute_trace_stats_on_extension && !client_computed_stats;
@@ -1529,7 +1535,8 @@ mod tests {
     #[tokio::test]
     #[allow(clippy::unwrap_used)]
     async fn test_compute_stats_tag_tracer_computes() {
-        // Tracer computed stats (Datadog-Client-Computed-Stats header set) → tag "0".
+        // Tracer computed stats (Datadog-Client-Computed-Stats header set) → tag "1".
+        // Backend still computes as a safety net; extension skips its own stats generation.
         check_compute_stats_behavior(false, true).await;
     }