src: add missing inialization in agent.h

This missing initialization was reported by the coverity scans
we are in the process of re-enabling.

Signed-off-by: Michael Dawson <[email protected]>

PR-URL: #40379
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Minwoo Jung <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>

src/tracing/agent.h

@@ -76,7 +76,7 @@ class AgentWriterHandle {
   inline AgentWriterHandle(Agent* agent, int id) : agent_(agent), id_(id) {}
 
   Agent* agent_ = nullptr;
-  int id_;
+  int id_ = 0;
 
   friend class Agent;
 };