convert string into an integer

if you're already familiar with auditd/auditctl rules, that's the way to go for that granular of an experience, but you would definitely want to configure syslog-ng to shoot your custom logging off-box as it will get very chatty; that IO overhead will impact system performance and fill your local disks quickly. Current rules on my 16.1.3 system:

[root@ltm3:Active:Standalone] rules.d # auditctl -l
-w /etc/selinux -p wa -k MAC-policy

Those can be modified (technically, I'd open a support case to inquire about supportability) but understand that any system patch/upgrade would wipe out that system configuration, so you'd need a process to make sure that persists.

tcpdump executions, however, are always already logged in /var/log/ltm:

[root@ltm3:Active:Standalone] auditd # cat /var/log/ltm | grep -i tcpdump
Nov 15 15:51:17 ltm3.test.local notice tmm[42230]: 013e0000:5: Tcpdump starting locally on 127.1.1.1:2 from 127.1.1.254:41639
Nov 15 15:51:17 ltm3.test.local notice tmm[42230]: 013e000b:5: Tcpdump starting DPT providers:Noise Provider
Nov 15 15:51:17 ltm3.test.local notice tmm1[42230]: 013e0000:5: Tcpdump starting locally on 127.1.1.2:2 from 127.1.1.254:41639
Nov 15 15:51:17 ltm3.test.local notice tmm1[42230]: 013e000b:5: Tcpdump starting DPT providers:Noise Provider
Nov 15 15:51:17 ltm3.test.local notice tmm[42230]: 013e0002:5: Tcpdump stopping on 127.1.1.1:2 from 127.1.1.254:41639
Nov 15 15:51:19 ltm3.test.local notice tmm1[42230]: 013e0002:5: Tcpdump stopping on 127.1.1.2:2 from 127.1.1.254:41639