Added max-conns annotation

[vrrs]

Proposed changes

These changes enable the annotation max-conns which set the parameter max_conns on upstream server in nginx config. For instance,
nginx.org/max-conns: 6 on ingress.yaml should add to ngnix config

upstream myService-ingress-prod-myService.production.ingress.net-myService-prod-80 {
   server 125.255.255.255:8888 max_fails=3 fail_timeout=10 max_conns=6;
}

Here it's the documentation for max_conn https://nginx.org/en/docs/http/ngx_http_upstream_module.html

max_conns=number
limits the maximum number of simultaneous active connections to the proxied server (1.11.5). Default value is zero, meaning there is no limit. If the server group does not reside in the shared memory, the limitation works per each worker process.

Use case

This parameter is very useful on load balancing long lived connections and limiting load on the destination servers.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[Dean-Coakley]

Hi @vrrs

Thanks a lot for taking the time to write this PR!

Just a few small changes required and we'll get this merged shortly!

• Add max-conns entry to annotation doc: https://github.com/nginxinc/kubernetes-ingress/blob/master/docs/configmap-and-annotations.md#backend-services-upstreams
• Necessary to add MaxConns to createUpstream of ingress.go for annotation to function correctly - https://github.com/nginxinc/kubernetes-ingress/blob/master/internal/configs/ingress.go#L291

[vrrs]

@Dean-Coakley Made the changes suggested.

[Dean-Coakley]

Lgtm once: #614 (comment) is applied

Lgtm

[pleshakov]

@vrrs looks good! ty
just a small doc problem left.

[pleshakov]

Small doc problem, the max-conns ConfigMap is not supported, so it has to be removed from the doc:

Suggested change

	| `nginx.org/max-conns` | `max-conns` | Sets the value of the [max_conns](https://nginx.org/en/docs/http/ngx_http_upstream_module.html#max_conns) parameter of the `server` directive. | `0` | |
	| `nginx.org/max-conns` | N/A | Sets the value of the [max_conns](https://nginx.org/en/docs/http/ngx_http_upstream_module.html#max_conns) parameter of the `server` directive. | `0` | |

[vrrs]

@pleshakov done