/* try MemAvailable (present since Linux 3.14), falling back to a calculation based on sysinfo() and Cached */

        if (NULL == (f = fopen("/proc/meminfo", "r")))
        {
                SET_MSG_RESULT(result, zbx_dsprintf(NULL, "Cannot open /proc/meminfo: %s", zbx_strerror(errno)));
                return SYSINFO_RET_FAIL;
        }

        if (FAIL == (res = byte_value_from_proc_file(f, "MemAvailable:", "Cached:", &value)))
        {
                SET_MSG_RESULT(result, zbx_strdup(NULL, "Cannot obtain the value of MemAvailable from /proc/meminfo."));
                goto close;
        }

        if (SUCCEED == res)
        {
                SET_UI64_RESULT(result, value);
                ret = SYSINFO_RET_OK;
                goto close;
        }

MemAvailable: An estimate of how much memory is available for starting new
              applications, without swapping. Calculated from MemFree,
              SReclaimable, the size of the file LRU lists, and the low
              watermarks in each zone.
              The estimate takes into account that the system needs some
              page cache to function well, and that not all reclaimable
              slab will be reclaimable, due to items being in use. The
              impact of those factors will vary from system to system.

Linux c7zs1.local 3.10.0-1062.12.1.el7.x86_64 #1 SMP Tue Feb 4 23:02:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
cat /proc/meminfo 
MemTotal:        1010080 kB
MemFree:           80260 kB
MemAvailable:     336324 kB
Buffers:               0 kB
Cached:           398228 kB
SwapCached:            0 kB

zabbix_agentd -t vm.memory.size[available]
vm.memory.size[available]                     [u|343908352]

### in kB: 335848

MemFree:           78964 kB
Buffers:               0 kB
Cached:           399228 kB

[root@m8]# uname -a
Linux m8.local 4.18.0-147.5.1.el8_1.x86_64 #1 SMP Wed Feb 5 02:00:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

[root@m8]# head -n 7 /proc/meminfo 
MemTotal:        2031268 kB
MemFree:         1235676 kB
MemAvailable:    1355472 kB
Buffers:            3292 kB
Cached:           240036 kB
SwapCached:            0 kB
Active:           538432 kB