Currently Node Exporter has a metric called `node_uname_info` which of
course exposes uname info. While this is nice, it does not help if you
are running different OSes which could have similar uname info.
Therefore parse `/etc/os-release` or `/usr/lib/os-release` and expose a
`node_os_info` metric which provide information regarding the OS
release/version of the node. Also expose the major.minor part of the OS
release version as `node_os_version`.
Since the os-release files will not change often, cache the parsed
content and only refresh the cache if the modification time changes.
This `os` collector will read files outside of `/proc` and `/sys`, but
the os-release file is widely used and the format is standardized:
https://www.freedesktop.org/software/systemd/man/os-release.html
Bug: https://github.com/prometheus/node_exporter/issues/1574
Signed-off-by: Benjamin Drung <benjamin.drung@ionos.com>
