monitoring: add fs_io_operations alert for gitserver, zoekt

Created by: bobheadxi

Add monitoring for gitserver and zoekt-* services read and write operations. We use a sum of reads and writes rates over 1h to get a picture of sustained IO usage, and set the alert very high since it's currently unclear how we might want to alert on this at the moment.

Closes https://github.com/sourcegraph/sourcegraph/issues/12163 , which also describes why we are using a sum of reads and writes here.

Queries:

• gitserver
• zoekt-indexserver
• zoekt-webserver