Add option to configure proxy via env

[Crited]

When trying to connect to Red Hat OpenShift, one has to directly access the provided API without port. Due to the fact, that the default port is set to 9090 and can't be blank it is not possible to use this plugin with RedHat OpenShift. For example:

Version: v0.1.0

For example:

sudo -u icinga ./check_prometheus -H ***.net query --query '***' --secure
[UNKNOWN] - Post "https://***.net:9090/api/v1/query": dial tcp ***:9090: connect: connection refused (*url.Error)

Should preferably just call http://***.net/api/v1/query because using 443 or 80 explicitly doesn't work.

sudo -u icinga ./check_prometheus -H ***.net query --query '***' -p 443 --secure
[UNKNOWN] - Post "https://***.net:443/api/v1/query": dial tcp ***:443: connect: connection refused (*url.Error)

[martialblog]

@Crited Not sure I understand the issue correctly. All variations of http/https and ports should be possible

# With defaults
check_prometheus -H localhost health
[UNKNOWN] - Could not get status: Get "http://localhost:9090/-/healthy": dial tcp 127.0.0.1:9090:

# With defaults and --secure
check_prometheus -H localhost health --secure
[UNKNOWN] - Could not get status: Get "https://localhost:9090/-/healthy": dial tcp 127.0.0.1:9090: 

check_prometheus -H localhost -p 443 health --insecure
[UNKNOWN] - Could not get status: Get "http://localhost:443/-/healthy": dial tcp 127.0.0.1:443: 

check_prometheus -H localhost -p 443 health --secure
[UNKNOWN] - Could not get status: Get "https://localhost:443/-/healthy": dial tcp 127.0.0.1:443: 

check_prometheus -H localhost -p 80 health --insecure
[UNKNOWN] - Could not get status: Get "http://localhost:80/-/healthy": dial tcp 127.0.0.1:80:

check_prometheus -H localhost -p 80 health --secure 
[UNKNOWN] - Could not get status: Get "https://localhost:80/-/healthy": dial tcp 127.0.0.1:80: 

[martialblog]

Talked internally. We assume it's a network issue due to proxies inbetween.

I assume that the issue is that the plugin currently doesn't support the Env Variables HTTP_PROXY, HTTPS_PROXY and NO_PROXY

Already started to fix it.