proposal: Introduce `metrics` to store stats

[Xuanwo]

I propose to introduce metrics to store our stats include:

Compile requests                    722
Compile requests executed           554
Cache hits                          550
Cache hits (C/C++)                  100
Cache hits (Rust)                   450
Cache misses                          0
Cache timeouts                        0
Cache read errors                     0
Forced recaches                       0
Cache write errors                    0
Compilation failures                  4
Cache errors                          0
Non-cacheable compilations            0
Non-cacheable calls                   4
Non-compilation calls                 4
Unsupported compiler calls            0
Average cache write               0.000 s
Average compiler                  0.000 s
Average cache read hit            0.000 s
Failed distributed compilations       0

Non-cacheable reasons:
crate-type                          1
unknown source language              
-                                     4
incremental                           2
-E                                    2

Goods

• Easier coding with increment_counter instead of RwLock across the whole code base by hand
• With prometheus recorder setup, our users can export those metrics to their prometheus server.

Bads

• A new dep
• Big refactor on current code

[Xuanwo]

I have tried this idea, and it's not good enough.