set user agent on scrape requests to nginx.

[asherf]

Proposed changes

This changes the User-Agent header that is being sent when calling nginx stats endpoint.
Currently, the User-Agent is the default Go Http client user agent string (Go-http-client/1.1), which is not very useful when looking at nginx logs.
This will change it to be NGINX-Prometheus-Exporter/v0.5.0 (with the version we get when building the docker image)
I have tested locally (with a custom docker image) in my k8s cluster (faked the version number when calling docker build).

(ignore the red color, this is just kubetail choosing random colors)

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

[pleshakov]

@asherf thanks for the PR!

I suggest implementing this feature using a custom RoundTripper -- https://godoc.org/net/http#RoundTripper.

There is an example implementation of a similar feature here https://github.com/kubernetes/kubernetes/blob/bc1d8c6d61078269540f807248a455cba09e672c/staging/src/k8s.io/client-go/transport/round_trippers.go#L144 Note that it is important to clone requests by doing a deep copy to adhere of the contract of the RoundTripper.

With a RoundTripper, we will only have to change the httpClient in exporter.go:

	httpClient := &http.Client{
		Timeout:   timeout.Duration,
		Transport: userAgentRT,
	}

where userAgentRT is a variable that holds a RoundTripper for setting the user-agent header

As the result, both clients for NGINX and NGINX Plus will set the custom user-agent header.

[asherf]

@pleshakov Thanks for that useful feedback! It took me a while (busy at work + I'm new to Go)
But I got this working.
Let me know if you have any further feedback since I am a Go noob.
I am happy to incorporate any feedback you have to this PR.

[pleshakov]

@asherf thanks! looks good! just a couple of small suggestions

I also requested a review from @Rulox

[Rulox]

lgtm

+1 to @pleshakov suggestions about readability and best practices.

Additionally, please rebase against master, there are conflicts in exporter.go due to a PR merged recently.

Thanks for the contribution !

[pleshakov]

@asherf thanks for making the changes. there a bug appeared after the PR was rebased against the master branch. otherwise, looks good!

[pleshakov]

👍