feat(memory_pool): add TrackConsumersPool::metrics() to expose cons…

[bert-beyondloops]

Which issue does this PR close?

• Closes Expose TrackConsumersPool memory consumer metrics programmatically #21146

Rationale for this change

There is currently no way to programmatically inspect the memory consumption of individual consumers tracked by TrackConsumersPool. The only available method, report_top(), returns a formatted string intended for human-readable output, making it unsuitable for programmatic use (e.g., metrics collection, monitoring, or custom reporting).

What changes are included in this PR?

Added a metrics() method to TrackConsumersPool that returns a Vec — a snapshot of all currently tracked consumers. Each MemoryConsumerMetrics entry exposes:

name — the consumer's name
can_spill — whether the consumer supports spilling to disk
reserved — current bytes reserved
peak — peak bytes reserved
This allows callers to inspect memory usage programmatically without parsing formatted strings.

Are these changes tested?

Yes. A dedicated unit test test_track_consumers_pool_metrics was added in pool.rs that verifies:

• An empty pool returns no metrics
• name, can_spill, reserved, and peak are correctly reported for each consumer
• Peak is tracked independently from current reservation (grow then shrink scenario)
• Dropped consumers are removed from metrics

Are there any user-facing changes?

No