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 <mdawson@devrus.com>

PR-URL: nodejs#40379
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: mhdawson

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;
 };