Update GetNetOverflow function to use nstat instead of netstat

[ja20222]

nstat is installed on linux by default so we can use that instead of relying on netstat.

To get the network overflow run the following command:

$ nstat -az | grep -i TcpExtListenOverflows

which will return the following output:

TcpExtListenOverflows           0                  0.0

The NGINX Agent should parse the particular output and populate the float64 :

agent/src/core/network/network.go

Line 59 in 709e402

func GetNetOverflow() (float64, error) {

Ensure consistent output - Rocky Linux 9 (x86_64) and AlmaLinux 8 (x86_64), Ubuntu 22, RHEL 8, SUSE 15, FreeBSD 13 (pasted in the PR output)

[Harshitsoni2000]

Hi @Dean-Coakley , @ja20222 . I would love to work on this issue. Could you throw some light on how I can begin?

[Dean-Coakley]

Hi @Harshitsoni2000

First I recommend following this section of our README to setup a development environment: https://github.com/nginx/agent#development-environment-setup

If that goes ok, some steps to get started:

1. Replace the usage of netstat command with nstat -az in

   agent/src/core/network/network.go

   Line 59 in 709e402

   func GetNetOverflow() (float64, error) {

2. Replace the regex to parse the new value correctly:

   agent/src/core/network/network.go

   Line 41 in 709e402

   reTimesOverflowed = regexp.MustCompile("times the listen queue of a socket overflowed")

3. Enable parsing of NetOverflow on linux