Add constLabels support via cli arg/env var

[Dean-Coakley]

Proposed changes

Adds support for specifying a list of constant labels that all the metrics of the exporter will have.

• Add supported cli arg prometheus.const-labels
• Add supported env var CONST_LABELS

Checklist

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

• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch on my own fork

[Rulox]

lgtm.

Have u tried what happens if you use NewNginxPlusCollector with constLabels=nil ? I think nothing, but just in case. Most of the time, users won't need to use these labels (or they will configure them in prometheus.yaml), this is for a different use case.

[pleshakov]

@Dean-Coakley thx. Please see my comments and suggestions. Also, don't forget to update the docs.

I wasn't able to configure the const lables using command line argument :( this might be my mistake though

[Dean-Coakley]

@Rulox Yeah. Nothing happens if constLabels is set to nil.

[pleshakov]

@Dean-Coakley thanks! looks good. Found a few new small problems.

[pleshakov]

@Dean-Coakley the latest changes lgtm!

During testing I discovered how the exporter handles invalid metrics labels:

./nginx-prometheus-exporter -nginx.scrape-uri=http://demo.nginx.com/stub_status -prometheus.const-labels="a=b,c a=d"
2019/12/09 13:02:24 Starting NGINX Prometheus Exporter Version=0.4.2 GitCommit=8e22a75
panic: descriptor Desc{fqName: "nginxexporter_build_info", help: "Exporter build information", constLabels: {}, variableLabels: []} is invalid: "c a" is not a valid label name for metric "nginxexporter_build_info"

goroutine 1 [running]:
github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(0xc0000aa3c0, 0xc0000f3d78, 0x1, 0x1)
        /Users/michael/work/nginx/prometheus/go/src/github.com/nginxinc/nginx-prometheus-exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:391 +0xad
main.main()
        /Users/michael/work/nginx/prometheus/go/src/github.com/nginxinc/nginx-prometheus-exporter/exporter.go:279 +0x464

The message doesn't look user friendly. I suggest we try to fix it. I'm ok if we do that in a separate PR.

Let me know your thoughts @Dean-Coakley @Rulox

[Dean-Coakley]

@pleshakov I will improve validation in this PR.