Fix memory leak in JfrMetadata::reset()

[jbachorik]

What does this PR do?:
JfrMetadata::reset() cleared _root._children/_root._attributes without deleting the heap-allocated Element/NoField objects initialize() had allocated, leaking them and leaving the tree vulnerable to use-after-free if the allocator later reused a freed address. Adds recursive deletion of the Element tree and the tracked _nofields list before clearing.

Motivation:
Only reachable via a profiler restart (reset() followed by initialize() a second time in the same process). Found while investigating PROF-15075 (SIGSEGV in Recording::writeElement) but not its cause — that crash trace shows a plain chunk dump with no restart involved. Closing this separately since it's a real leak/UAF risk on its own.

How to test the change?:

• ddprof-lib/src/test/cpp/jfrMetadata_ut.cpp exercises initialize()/reset() cycles, including with context attributes, and asserts no leaked/dangling Elements.

For Datadog employees:

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• This PR doesn't touch any of that.
• JIRA: [JIRA-XXXX]

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[dd-octo-sts]

CI Test Results

Run: #30355775898 | Commit: ca6c687 | Duration: 17m 59s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

---

Updated: 2026-07-28 12:14:17 UTC

[dd-octo-sts]

Benchmark Results (commit 008d688)

Pipeline: https://gitlab.ddbuild.io/DataDog/apm-reliability/benchmarking-platform/-/pipelines/127401287 Commit: 008d6889e4f9acc4be699bb2ac845f3396e20528

⚠️ Significant outliers

• 🟢 fj-kmeans (JDK 21): runtime -3.9% (2745→2638 ms)
• 🟢 future-genetic (JDK 21): runtime -3% (2122→2058 ms)

Runtime details (per benchmark × JDK)

Benchmark	JDK	Latest	Dev	Δ (dev vs latest)	Issues L/D
akka-uct	21	✅ 10193 ms (21 iters)	✅ 10416 ms (21 iters)	≈ +2.2% (±11.2%)	— / —
akka-uct	25	✅ 8880 ms (24 iters)	✅ 8743 ms (24 iters)	≈ -1.5% (±10%)	— / —
finagle-chirper	21	✅ 6002 ms (33 iters)	✅ 5929 ms (33 iters)	≈ -1.2% (±24.7%)	⚠️ W:3 / ⚠️ W:3
finagle-chirper	25	✅ 5437 ms (36 iters)	✅ 5445 ms (36 iters)	≈ +0.1% (±24.3%)	⚠️ W:3 / ⚠️ W:3
fj-kmeans	21	✅ 2745 ms (68 iters)	✅ 2638 ms (72 iters)	🟢 -3.9%	— / —
fj-kmeans	25	✅ 2832 ms (66 iters)	✅ 2832 ms (66 iters)	≈ 0% (±2.6%)	— / —
future-genetic	21	✅ 2122 ms (87 iters)	✅ 2058 ms (90 iters)	🟢 -3%	— / —
future-genetic	25	✅ 2059 ms (90 iters)	✅ 2049 ms (90 iters)	≈ -0.5% (±2.6%)	— / —
naive-bayes	21	✅ 1257 ms (136 iters)	✅ 1304 ms (132 iters)	≈ +3.7% (±33.1%)	— / —
naive-bayes	25	✅ 1028 ms (167 iters)	✅ 1014 ms (169 iters)	≈ -1.4% (±31.8%)	— / —
reactors	21	✅ 16668 ms (15 iters)	✅ 15846 ms (15 iters)	≈ -4.9% (±7.4%)	— / —
reactors	25	✅ 18281 ms (15 iters)	✅ 18391 ms (15 iters)	≈ +0.6% (±5%)	— / —

Internal counter details (ddprof)

ddprof internal counters, latest / dev (✅ = 0, · = unavailable):

Benchmark	JDK	Dropped rec	Dropped jvmti	Dropped trace	Skipped WC	AGCT fail	Unwind fail
akka-uct	21	✅ / ✅	✅ / ✅	4 / 2	1961 / 1939	✅ / ✅	✅ / ✅
akka-uct	25	✅ / ✅	✅ / ✅	6 / 1	2369 / 2031	✅ / ✅	✅ / ✅
finagle-chirper	21	✅ / ✅	✅ / ✅	4 / 1	8227 / 8628	✅ / ✅	✅ / ✅
finagle-chirper	25	✅ / ✅	✅ / ✅	✅ / ✅	8367 / 8248	✅ / ✅	✅ / ✅
fj-kmeans	21	✅ / ✅	✅ / ✅	2 / 1	1267 / 1276	✅ / ✅	✅ / ✅
fj-kmeans	25	✅ / ✅	✅ / ✅	✅ / ✅	1287 / 1259	✅ / ✅	✅ / ✅
future-genetic	21	✅ / ✅	✅ / ✅	2 / 1	2932 / 3002	✅ / ✅	✅ / ✅
future-genetic	25	✅ / ✅	✅ / ✅	2 / 2	2843 / 2845	✅ / ✅	✅ / ✅
naive-bayes	21	✅ / ✅	✅ / ✅	2 / 4	3484 / 3550	✅ / ✅	✅ / ✅
naive-bayes	25	✅ / ✅	✅ / ✅	2 / 4	3492 / 3488	✅ / ✅	✅ / ✅
reactors	21	✅ / ✅	✅ / ✅	1 / 2	1638 / 1621	✅ / ✅	✅ / ✅
reactors	25	✅ / ✅	✅ / ✅	1 / 1	1843 / 1824	✅ / ✅	✅ / ✅