refactor(profiling): scope the OTEP-4947 start-up probe ids to the probe

canInstallContext() runs exactly once per process, so holding its two ids at
module scope kept them live for the process lifetime to save an allocation that
never repeats. As locals they are collectable as soon as the probe returns.

Author: szegedi

packages/dd-trace/src/otel-thread-ctx.js

@@ -256,11 +256,6 @@ function missingApiMember (ns) {
   }
 }
 
-// Zero-filled ids for the probe below. A record is only ever readable while it is
-// installed, and an all-zero trace id identifies nothing.
-const PROBE_TRACE_ID = new Uint8Array(16)
-const PROBE_SPAN_ID = new Uint8Array(8)
-
 // Install and detach one throwaway context, to establish that this process can
 // actually do it before any span depends on it.
 //
@@ -275,7 +270,10 @@ const PROBE_SPAN_ID = new Uint8Array(8)
 // declining to start is still an option.
 function canInstallContext (ns) {
   try {
-    const probe = new ns.ThreadContext(PROBE_TRACE_ID, PROBE_SPAN_ID)
+    // Zero-filled ids: the record is only readable while it is installed, which
+    // is for the length of this function, and an all-zero trace id identifies
+    // nothing.
+    const probe = new ns.ThreadContext(new Uint8Array(16), new Uint8Array(8))
     probe.enter()
     ns.clearContext()
     return true