Use the unified Hashtable API in client-side stats

[dougqh]

Stacked on #12101. Review that first — this PR's diff against it is just AggregateTable.

What Does This Do?

Migrates client-side statistics off the deprecated Hashtable.Support facade and onto the API from #12101, as the first real consumer of it.

This is the "use it to review it" half of that PR. The API was shaped in the abstract; this is what it looks like from the outside.

Result

AggregateTable goes from 164 to 132 lines, and from four pieces of hand-managed state to one:

// before
private final Hashtable.Entry[] buckets;
private final int maxAggregates;
private int size;
private int evictCursor;

// after
private final Hashtable.State<AggregateEntry> state;

The biggest deletion is evictOneStale and its helper — a cursor-resumed two-pass scan over [cursor, length) then [0, cursor), ~25 lines — which collapses into one call that reserves a slot and evicts only if the table is actually full:

if (!Hashtable.tryReserveOrEvict(state, STALE)) {
  return null;                        // full, nothing stale -- drop the datum
}

The caller no longer knows a cursor exists, but still gets its amortization: a sustained eviction stream never re-scans the hot prefix more than twice across N evictions.

Also gone: the manual size-- after every unlink (the manager owns the count now), the three paired resets in clear, and the iterator loop in expungeStaleAggregates.

STALE is a static final Predicate<AggregateEntry> — non-capturing, so eviction allocates nothing, and typed, so it needs no cast.

What this exercised in the API

Two warts found by writing this, both fixed in #12101 rather than worked around here:

• removeMatching needed an explicit Hashtable.<AggregateEntry> type witness, because TEntry had nothing to infer from.
• Eviction predicates took Predicate<? super Entry>, forcing entry -> ((AggregateEntry) entry).getHitCount() == 0.

Parameterizing State<TEntry> and having the statics take it fixed both, and made a spine/manager mismatch unconstructible rather than merely discouraged.

Behavior

Unchanged. Same cap, same evict-a-stale-entry-or-drop policy on the miss path, same amortized resumable scan — the scan just lives in the primitive now.

Follow-up this unblocks

No production code references Hashtable.Support any more. The facade holds no logic and can be deleted outright — deliberately left for its own PR so this one stays a behaviour-neutral migration.

Test plan

• ./gradlew :dd-trace-core:test --tests "datadog.trace.common.metrics.*" — passes unchanged
• ./gradlew :dd-trace-core:compileJava — no deprecation warnings from AggregateTable any more

🤖 Generated with Claude Code

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.80 s	14.72 s	[-0.3%; +1.3%] (no difference)
startup:insecure-bank:tracing:Agent	13.64 s	13.69 s	[-1.1%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	17.47 s	17.38 s	[-0.4%; +1.4%] (no difference)
startup:petclinic:iast:Agent	17.52 s	17.61 s	[-1.4%; +0.4%] (no difference)
startup:petclinic:profiling:Agent	16.80 s	17.24 s	[-6.8%; +1.7%] (no difference)
startup:petclinic:sca:Agent	17.54 s	17.39 s	[-0.0%; +1.8%] (no difference)
startup:petclinic:tracing:Agent	16.67 s	16.79 s	[-1.8%; +0.4%] (no difference)

Commit: 8435b639 · CI Pipeline · Benchmarking Platform UI

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