process: handle possible fio.read errors

Once I catched an error during tests:
```
tarantool/override/metrics/psutils/psutils_linux.lua:49: bad argument #1 to 'string.split' (string expected, got nil)
```

That shows that in some rare cases fio.read can return `nil, err` but
before this patch this case was unhandled. This patch fixes it.

Also it was pretty strange to print a message via "print" function. It was changed to "log" module
that is more native for tarantool.

This patch doesn't contain tests because it's pretty hard to reproduce such error (file was opened
but `file:read()` returned an errors for some reasons).

Closes #526

Author: olegrok

CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Fixed a potential `process` metrics error when `fio.read` returns an empty string or an error.
+
 # [1.5.0] - 2025-08-13
 
 ### Added

metrics/psutils/psutils_linux.lua

@@ -1,6 +1,7 @@
 local fio = require('fio')
 local string = require('string')
 local ffi = require('ffi')
+local log = require('log')
 
 local get_nprocs_conf = function() end
 if jit.os == 'Linux' then
@@ -19,7 +20,7 @@ local function get_cpu_time()
 
     local stats_raw = cpu_stat_file:read(512)
     cpu_stat_file:close()
-    if #stats_raw == 0 then
+    if stats_raw == nil or #stats_raw == 0 then
         return nil
     end
 
@@ -37,14 +38,22 @@ local function get_cpu_time()
 end
 
 local function parse_process_stat(path)
-    local stat = fio.open(path, 'O_RDONLY')
-    if stat == nil then
-        print('stat open error')
+    local stat, err = fio.open(path, 'O_RDONLY')
+    if err ~= nil then
+        log.error('stat open error: %s', tostring(err))
         return nil
     end
 
-    local s = stat:read(512)
+    local s
+    s, err = stat:read(512)
     stat:close()
+    if err ~= nil then
+        log.error('stat read error: %s', tostring(err))
+        return nil
+    end
+    if s == nil or #s == 0 then
+        return nil
+    end
 
     local stats = string.split(s)
     return {