Add mempool metrics related to memory usage

[conradoplg]

Motivation

It would be good to have metrics that report the memory usage of the mempool. One of these is zcash.mempool.usage.bytes that zcashd uses.

However, that's a bit tricky to measure. We can use https://github.com/str4d/memuse but we'd still need to implement a trait for every type used by the mempool, or preferably adding a derive macro to that package.

Specifications

Designs

Related Work

This is a follow-up to #2626, and was postponed due to additional work required.

[teor2345]

We're not planning on adding more metrics soon.