added Host header option

[mariovitale1979]

Proposed changes

In my setup using Bunkerweb, the 'internal' HTTP calls to the stub_status endpoint where blocked by the different security features of Bunkerweb. Only by specifying the Host in the header, nginx_exporter was able to reach the stub_status endpoint

[nginx-bot]

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[github-actions]

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[mariovitale1979]

I have hereby read the F5 CLA and agree to its terms

[vepatel]

hi @mariovitale1979, this PR is introducing breaking changes to NewNginxClient so anyone importing that function might be affected.

can you please create a gh issue clearly stating

• What is the exact requirement
• How this PR is addressing the issue

[mariovitale1979]

Well, let me try to explain what issue I had to solve for my setup.
My web-applications are managed by a docker-compose file and as webserver I don't use native nginx but bunkerweb (see https://github.com/bunkerity/bunkerweb) which is basically an nginx wrapper with a bunch of extra security features (ModSecurity, bad behaviour detection, etc etc)
When installing and configuring nginx-prometheus-exporter as documented, I noticed that the call made by nginx-prometheus-exporter to the stub_status endpoint always returned an HTTP 403 error. Problem was not because I didn't allow certain network/ip range. Only solution for me was to add the 'Host' parameter in the Header sent out by nginx-prometheus-exporter.
I'm not a nginx expert but for Bunkerweb that is normal, you you can only reach the stub_status endpoint by doing HTTP calls to a servername that is declared in the bunkerweb config

I hope my explanation helps a bit and I completely admit that maybe this solution is only needed for my particular setup and/or maybe there is a solution that didn't require me to fork this project but after almost 1 full day investigating, i settled with this solution.