Add a metric containing the size of generated documentation

[Kobzol]

This week's triage report said something like "we don't track the size of generated documentation in rustc-perf". Well, I figured, why don't we? It seems easy enough.

CC @GuillaumeGomez @jsha Is this metric interesting for rustdoc developers? Currently I store the size of the whole target/doc directory, should we store something else?

[GuillaumeGomez]

It sounds like a very good idea! I think storing the number of file and also the mean (total size divided by number of files) would help us have a good idea of how things are evolving.

[Kobzol]

I also thought about storing the number of files, that sounds useful.

But calculating some derived data (like the mean) is a bit out of scope I think. We already have ~20 metrics and I don't think that we should add derived calculations to them, it seems like we should add some other approach for that (maybe just calculating it in the dashboard on demand). What do you think @Mark-Simulacrum?

[GuillaumeGomez]

If we store both the number of files and the total size, there is no need to store the mean, it's easy to compute on demand.

[Mark-Simulacrum]

Definitely agree with not storing it - and not quite sure it makes sense for the dashboard to compute, but obviously something that users can pretty easily do themselves.

I think number of files is unlikely to be too interesting (our benchmarks are largely constant so I'm not sure how often we're going to see a shift there), but storing it seems fine.

[Kobzol]

@GuillaumeGomez Does rustdoc perform changes that vary the number of files (for a documented crate that stays the same) often enough that it's worth it to save the count?

[Kobzol]

I added also file count tracking, it's simple enough since we already gather the file size. I'm not sure if the size: prefix makes sense for the file count metric.

[GuillaumeGomez]

@GuillaumeGomez Does rustdoc perform changes that vary the number of files (for a documented crate that stays the same) often enough that it's worth it to save the count?

No but new types gets added in the std/core libraries. Considering the total number of files, it shouldn't matter much but it's better to be able from one look to tell why there is a small change in the total size.

[rylev]

I have one small nit, but otherwise looks great!

[Kobzol]

@Mark-Simulacrum @GuillaumeGomez any further comments? :)

[GuillaumeGomez]

Looks good to me!

[rylev]

Looks good!