Track memory usage in `MemUsageCollector`

[waynexia]

Description

We have implemented a MemUsageCollector in https://github.com/CeresDB/ceresdb/blob/6b929ca19a/analytic_engine/src/instance/mem_collector.rs but no one reports memory consumptions to it.

One of the use cases is deciding whether to schedule a flush job based on memory usage: https://github.com/CeresDB/ceresdb/blob/6b929ca19abf48ba9afd1cb554ec9a25c7783a42/analytic_engine/src/instance/write.rs#L307-L331

Proposal

Tracking allocation using this collector. For now I think tracks memtable is enough

Additional context