[RemoteCacheMetrics] Fix percentage bug and improve stats

[saraadams]

This change

• fixes the percentage shown, it was showing the inverse value (cache miss % instead of cache hit %)
• fixes the "remote upload outputs" time, which mistakenly also included upload times of inputs (for RE).
• filters out local actions that don't do remote cache checks, as these are not relevant and falsify the %
• adds absolute numbers for how many cache checks were performed and how many were misses
• adds some documentation on disk_cache also being a remote cache

Contributes to #90

[saraadams]

Output before:

RemoteCacheMetrics: Collection of remote cache related metrics.
Total Remote Cache Check Duration: 24m 41s
Total Remote Download Outputs:     1h 1m 30s
Total Remote Upload Outputs:       16s
Percent cached remotely:           5,08%

Output after:

RemoteCacheMetrics: Metrics on the remote caching used. This includes both the use of `--remote_cache` and `--disk_cache`.
Number of cache checks:              12917
Number of cache misses:                921
Cache hit percentage:               92,87%
Time spend checking for cache hits: 24m 41s
Time spend downloading outputs:     1h 1m 30s
Time spend uploading outputs:       10s