fix(trace-stats): leave lang and tracerVersion empty for agent to populate

The CSS spec states these fields should be left empty in
ClientStatsPayload for the agent to populate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: VianneyRuhlmann

libdd-data-pipeline/src/stats_exporter.rs

@@ -164,16 +164,16 @@ fn encode_stats_payload(
         } else {
             meta.env.clone()
         },
-        lang: meta.language.clone(),
         version: meta.app_version.clone(),
         runtime_id: meta.runtime_id.clone(),
-        tracer_version: meta.tracer_version.clone(),
         sequence,
         stats: buckets,
         git_commit_sha: meta.git_commit_sha.clone(),
         process_tags: meta.process_tags.clone(),
-        // These fields are unused or will be set by the Agent
+        // These fields will be set by the Agent
         service: meta.service.clone(),
+        lang: String::new(),
+        tracer_version: String::new(),
         container_id: String::new(),
         tags: Vec::new(),
         agent_aggregation: String::new(),