fix(traces): restore _dd.compute_stats on span meta to fix missing trace.aws.lambda.hits

PR #1118 moved `_dd.compute_stats` from each span's `meta` (applied via
`get_tags_map()` / `ChunkProcessor`) to the payload-level `tracer_payload.tags`
only. The Datadog APM backend reads this signal from the root span's `meta`
field, not from the payload-level tags, so it stopped computing stats and
`trace.aws.lambda.hits` disappeared from the trace explorer.

Fix: propagate `_dd.compute_stats` to every span's `meta` in the same loop
that writes it to `tracer_payload.tags`, restoring the pre-#1118 behavior
while keeping the payload-level tag as well.

Also add debug logging to `handle_traces` to surface the header tags on
every incoming trace request, which aids diagnosing similar issues.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lym953

bottlecap/src/traces/trace_agent.rs

@@ -323,6 +323,7 @@ impl TraceAgent {
     }
 
     async fn stats(State(state): State<StatsState>, request: Request) -> Response {
+        debug!("TRACE_AGENT | stats request: {request:?}");
         match state
             .stats_processor
             .process_stats(request, state.stats_tx)
@@ -504,6 +505,8 @@ impl TraceAgent {
         let tracer_header_tags: libdd_trace_utils::tracer_header_tags::TracerHeaderTags<'_> =
             (&parts.headers).into();
 
+        debug!("TRACE_AGENT | tracer_header_tags: {tracer_header_tags:?}");
+
         let (body_size, mut traces): (usize, Vec<Vec<pb::Span>>) = match version {
             ApiVersion::V04 => {
                 match trace_utils::get_traces_from_request_body(http_common::Body::from_bytes(body))

bottlecap/src/traces/trace_processor.rs

@@ -360,9 +360,20 @@ impl TraceProcessor for ServerlessTraceProcessor {
                 } else {
                     "0"
                 };
+                debug!("TRACE_PROCESSOR | header_tags.client_computed_stats: {}, compute_stats: {}", header_tags.client_computed_stats, compute_stats);
                 tracer_payload
                     .tags
                     .insert(COMPUTE_STATS_KEY.to_string(), compute_stats.to_string());
+                // Also propagate to each span's meta so backends that read _dd.compute_stats
+                // from the root span's meta field (rather than the payload-level tags) still
+                // work correctly. This restores the pre-#1118 behavior where the tag was
+                // applied to every span via get_tags_map() / ChunkProcessor.
+                for chunk in &mut tracer_payload.chunks {
+                    for span in &mut chunk.spans {
+                        span.meta
+                            .insert(COMPUTE_STATS_KEY.to_string(), compute_stats.to_string());
+                    }
+                }
             }
         }
         let endpoint = Endpoint {
@@ -1513,9 +1524,21 @@ mod tests {
                     .tags
                     .get(crate::tags::lambda::tags::COMPUTE_STATS_KEY),
                 Some(&expected_tag.to_string()),
-                "_dd.compute_stats must be {expected_tag} (compute_trace_stats_on_extension={compute_trace_stats_on_extension}, \
+                "_dd.compute_stats must be {expected_tag} in payload.tags (compute_trace_stats_on_extension={compute_trace_stats_on_extension}, \
                  client_computed_stats={client_computed_stats})"
             );
+            // Also verify _dd.compute_stats is set on each span's meta so the backend
+            // can read it from the root span.
+            for chunk in &payload.chunks {
+                for span in &chunk.spans {
+                    assert_eq!(
+                        span.meta.get(crate::tags::lambda::tags::COMPUTE_STATS_KEY),
+                        Some(&expected_tag.to_string()),
+                        "_dd.compute_stats must be {expected_tag} in span.meta (compute_trace_stats_on_extension={compute_trace_stats_on_extension}, \
+                         client_computed_stats={client_computed_stats})"
+                    );
+                }
+            }
         }
     }