Add allowlisted metadata-only quota telemetry

[arkmish]

Description

Expose recorded namespace quota usage and limits for monitoring without recursively scanning the live data tree or changing quota enforcement.

The new AdminServer command is:

/commands/quota_stats?path=/app

Requests require an exact match in zookeeper.quotaStats.allowedNamespaces, a strictly validated JSON array that defaults to [].

• Validate the request path and the entire allowlist before accessing the tree. Root and reserved /zookeeper paths are rejected.
• Read only the namespace's existence and its exact existing quota-stat/limit nodes. Do not substitute an ancestor quota or recount the subtree.
• Return schema version, path, recorded count/byte usage and limits, plus explicit available and reason fields.
• Distinguish missing, incomplete, malformed, or changed metadata. Unavailable samples have null numeric fields; native unset limits are null, while configured zero remains zero.
• Use standard command errors for invalid requests/configuration and avoid echoing raw invalid metadata.

The allowlist limits which paths can be disclosed; it does not replace AdminServer access controls. Sampling remains non-atomic and may reflect accounting lag.

Tests

Validated CommandsTest, DataTreeTest, ServerMetricsTest, and ZooKeeperQuotaTest using Maven on JDK 11 with Java 8 API targeting:

Tests run: 76, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Coverage includes exact JSON/null semantics, strict path/allowlist validation, numeric bounds, missing and malformed quota records, no ancestor substitution, bounded read-only access, concurrent changes, namespace removal, and server restarts.

• Local code review completed

Changes that Break Backward Compatibility (Optional)

No client protocol, ACL decision, persistence-format, quota-enforcement, or write-accounting change. The command is default-deny and supplies no production namespace approvals.

available=true means the required metadata was readable and valid for the sample. It does not establish freshness, atomic consistency, safe capacity, or an approved operational budget.

Documentation (Optional)

Updated zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md with configuration syntax, command examples, response/error schemas, and sampling limitations.

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