Move memory stats gathering from polkadot to parity-util-mem

[koute]

I want to switch the jemalloc crate from jemallocator to tikv-jemallocator, however polkadot directly depends on the jemalloc-ctl crate which makes such a switch a breaking change (since jemalloc-ctl pulls the original jemalloc-sys, and you can't have two jemallocs compiled in at the same time).

So this moves that jemalloc-specific chunk of code (originally introduced in paritytech/polkadot#3612) from polkadot here, and abstracts it away.

[drahnr]

LGTM!

[ordian]

Thanks! Could you also update the changelog in parity-util-mem?

[koute]

Thanks! Could you also update the changelog in parity-util-mem?

Sure thing!

[dvdplm]

There are a few undocumented items and I think I agree with @ordian about preferring an Err to a panic.

[drahnr]

There are a few undocumented items and I think I agree with @ordian about preferring an Err to a panic.

Since one cannot create the item itself (fn new() always returns Err), I think a panic is fine, maybe a unreachable!() instead though

[koute]

There are a few undocumented items

If by undocumented you mean the lack of doc comments then yes, these are deliberately undocumented since the documentation's supposed to be on the facade (since that's what's supposed to define the public API; not much point in copy-pasting exactly the same thing for crate internal structures).

and I think I agree with @ordian about preferring an Err to a panic.

Well, it's not supposed to be ever called, which is why I think an unimplemented is apt there, but I don't really have a strong opinion about this so whatever you decide I'm fine with it.

[ordian]

FWIW, I'm fine with the current impl, was reviewing it in the pre-coffee state at first 😅

[koute]

Okay, I've updated the changelog (and I've also backfilled the release date of the 0.10.0). I've also put today's date for 0.10.1, since I'd like to immediately release it and whip up a polkadot PR to align it to these changes.

If you'd like anything else changed now's the time to speak up. (:

[ordian]

I've also put today's date for 0.10.1, since I'd like to immediately release it and whip up a polkadot PR to align it to these changes.

Sorry, do you want me to publish it? Maybe I misinterpreted your message. But feel free to do it, you should have the rights to publish to crates.io.

[koute]

Sorry, no, I'll do it myself next week; I was just busy with the RPC memory leak, which is why I didn't do it immediately. (: