feat(libdd-trace-utils): check for empty value in header datadog-client-computed-stats (#1900)

# What does this PR do?

Treat an empty string as a falsy value for the `Datadog-Client-Computed-Stats` header.

# Motivation

We want to support agent computed stats in the Serverless Compatibility Layer. Currently when the `Datadog-Client-Computed-Stats` header is sent it always disables agent computed stats, even when the value of the header is an empty string.

https://datadoghq.atlassian.net/browse/SVLS-8789

# Additional Notes

- [Java Tracer sends an empty string](https://github.com/DataDog/dd-trace-java/blob/6e28457d70c41bf847c4af7b3ba4e2f6c1371070/dd-trace-core/src/main/java/datadog/trace/common/writer/ddagent/DDAgentApi.java#L110-L117) for `Datadog-Client-Computed-Stats` header where other tracers omit the header altogether
- Go Agent [treats an empty string as a falsy value](https://github.com/DataDog/datadog-agent/blob/76aff83162011a15e5ee50295ac835f708e8ffa9/pkg/trace/api/api.go#L1049)
- See DataDog/serverless-components#51 for adding agent computed stats in the Serverless Compatibility Layer.

# How to test the change?

Added a debug log in a test build:

Before change with `DD_TRACE_STATS_COMPUTATION_ENABLED=false`
```
DEBUG datadog_trace_agent::trace_processor: Resolved tracer header tags: TracerHeaderTags { lang: "java", lang_version: "21.0.6", lang_interpreter: "OpenJDK 64-Bit Server VM", lang_vendor: "Microsoft", tracer_version: "1.61.1~e32291a78b", container_id: "", client_computed_top_level: true, client_computed_stats: true, dropped_p0_traces: 0, dropped_p0_spans: 0 }
```

After change with `DD_TRACE_STATS_COMPUTATION_ENABLED=false`
```
DEBUG datadog_trace_agent::trace_processor: Resolved tracer header tags: TracerHeaderTags { lang: "java", lang_version: "21.0.6", lang_interpreter: "OpenJDK 64-Bit Server VM", lang_vendor: "Microsoft", tracer_version: "1.61.1~e32291a78b", container_id: "", client_computed_top_level: true, client_computed_stats: false, dropped_p0_traces: 0, dropped_p0_spans: 0 }
```


Co-authored-by: duncan.harvey <duncan.harvey@datadoghq.com>

Author: duncanpharvey

libdd-trace-utils/src/tracer_header_tags.rs

@@ -27,11 +27,11 @@ pub struct TracerHeaderTags<'a> {
     pub lang_vendor: &'a str,
     pub tracer_version: &'a str,
     pub container_id: &'a str,
-    // specifies that the client has marked top-level spans, when set. Any non-empty value will
-    // mean 'yes'.
+    // specifies that the client has marked top-level spans, when set. If the header is present
+    // this value will resolve to 'true'
     pub client_computed_top_level: bool,
-    // specifies whether the client has computed stats so that the agent doesn't have to. Any
-    // non-empty value will mean 'yes'.
+    // specifies whether the client has computed stats so that the agent doesn't have to. If the
+    // header is present and is non-empty this value will resolve to 'true'
     pub client_computed_stats: bool,
     // number of trace chunks dropped in the tracer
     pub dropped_p0_traces: usize,
@@ -133,8 +133,8 @@ impl<'a> From<&'a HeaderMap<HeaderValue>> for TracerHeaderTags<'a> {
         if headers.get("datadog-client-computed-top-level").is_some() {
             tags.client_computed_top_level = true;
         }
-        if headers.get("datadog-client-computed-stats").is_some() {
-            tags.client_computed_stats = true;
+        if let Some(v) = headers.get("datadog-client-computed-stats") {
+            tags.client_computed_stats = !v.to_str().unwrap_or_default().is_empty();
         }
         if let Some(count) = headers.get("datadog-client-dropped-p0-traces") {
             tags.dropped_p0_traces = count
@@ -262,4 +262,26 @@ mod tests {
         assert_eq!(tags.dropped_p0_traces, 12);
         assert_eq!(tags.dropped_p0_spans, 0);
     }
+
+    #[test]
+    fn test_header_map_to_tags_computed_stats_empty_string() {
+        let val = "";
+        let mut header_map = HeaderMap::new();
+        header_map.insert("datadog-client-computed-stats", val.parse().unwrap());
+        let tags: TracerHeaderTags = (&header_map).into();
+        assert!(
+            !tags.client_computed_stats,
+            "expected client_computed_stats=false for datadog-client-computed-stats header value {val:?}"
+        );
+    }
+
+    #[test]
+    fn test_header_map_to_tags_computed_stats_not_set() {
+        let header_map = HeaderMap::new();
+        let tags: TracerHeaderTags = (&header_map).into();
+        assert!(
+            !tags.client_computed_stats,
+            "expected client_computed_stats=false when datadog-client-computed-stats header is not set"
+        );
+    }
 }