obs(api): no metrics on global expiration ZSET size or evictor sweep latency

Background

`sandbox:storage:global:expiration` is a single global ZSET shared by every
sandbox across all teams. The evictor calls `ExpiredItems` every 50 ms
(`pollInterval`), which issues a `ZRANGEBYSCORE` against this key on every tick.

Problem

There are currently no metrics to observe:

1. ZSET cardinality over time — no way to know whether
   `sandbox:storage:global:expiration` is growing unboundedly, shrinking after
   orphan sweeps (#3567), or stable at steady state.
2. Evictor sweep duration — no way to know how long each
   `ZRANGEBYSCORE + MGET pipeline` takes, or whether sweep latency is
   increasing as the ZSET grows.

Without these two signals it is impossible to:

• Set an alert before the ZSET grows large enough to cause latency spikes
• Verify that orphan cleanup (#3567) or future sharding fixes are actually
  reducing cardinality
• Correlate Redis command latency spikes with evictor tick cost

Production evidence

Observed 2026-08-14 09:00 – 09:30 (Redis big-key scan):

At `pollInterval = 50 ms` and N API allocations, the evictor issues
20 × N `ZRANGEBYSCORE` calls per second against the 22 k-member ZSET.
This load is currently invisible — there is no metric showing either the
key size trend or the per-sweep cost.

Observability gap

Why a gauge for size, not sampled on every evictor tick?

`global:expiration` is already the key being diagnosed as a high-frequency
hot key. Sampling `ZCARD` on every 50 ms evictor tick would add 20 × N
extra Redis calls per second — worsening the problem while observing it.
A single `ZCARD` piggybacked on the existing heal pass (every 5 min,
same cadence as #3604) adds negligible overhead and is sufficient for
capacity planning and alerting.

Why sweep duration matters

`ZRANGEBYSCORE` is O(log N + K) where N = 22,458 and K = batch size (256).
As N grows the per-sweep cost grows. Correlating sweep duration with ZSET
size over time makes the relationship visible before it becomes a latency
incident.

Related

• #3566 / #3567 — stale team-index entries that may be inflating
  `global:expiration` via orphaned ZSET members
• #3602 / #3603 — `cjson.decode` blocking Redis event loop on
  `startTransitionScript` (separate hot path)
• #3604 — per-team index SET size histogram (same observability series)
• #3591 — RFC: evaluate DragonflyDB as caching backend; ZSET size and
  sweep latency metrics would provide the baseline needed to evaluate
  whether a backend switch is warranted

/cc @jakubno @dobrac @ValentaTomas

Start by tracing the healExpirationIndex pass and the ExpiredItems call in the evictor, then identify the existing metrics conventions used by the API storage code. Add the proposed expiration_index.size gauge during the heal pass and sweep_duration histogram per ExpiredItems call, and verify both signals are emitted with the documented names and units.