Add hits/misses ratio to stats

[elichai]

Hi,
This adds a ratio to the stats. it looks like this:

$ ./target/debug/sccache -s
Compile requests                    862
Compile requests executed           648
Cache hits                          239
Cache hits (Rust)                   239
Cache misses                        409
Cache misses (Rust)                 409
Cache hits rate                   36.88 %
Cache hits rate (Rust)            36.88 %
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  0
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                 211
Non-compilation calls                 0
Unsupported compiler calls            3
Average cache write               0.000 s
Average cache read miss           1.158 s
Average cache read hit            0.002 s
Failed distributed compilations       0

Non-cacheable reasons:
crate-type                          137
incremental                          50
-                                    16
-o                                    8

Cache location                  Local disk: "/home/elichai2/.cache/sccache"
Cache size                            8 GiB
Max cache size                       20 GiB

[glandium]

I don't think this would work in the case where there are only hits or misses for a given language. You'd need to enumerate the intersection of all languages in hits and misses.

[glandium]

Following the style of other macros, this should be set_percentage_stat (also, you have a typo on percentage).

[sylvestre]

inactive, closing