Add internal setup package for managing file loading and registration

[stephen-soltesz]

This package will replace the registration logic in bigquery_exporter/main.go to support file reload and and prometheus registration.

Basically, the setup.File methods support writing a register-update loop like:

modified, err := f.IsModified()
if modified && err == nil {
    c := sql.NewCollector(...)
    err = f.Register(c)
} else {
    err = f.Update()
}

---

This change is 

[pboothe]

These changes look good to me with one small comment.

, approval

That said, they do work in further support of a more-risky deployment pattern where the underlying config file can change out from underneath the running binary. It would be nice to make sure that the idea of deploying configs and binaries together as a seamless whole was on the future roadmap for most of our systems, like we do with k8s-support for the platform.

Reviewable status: complete! 1 of 1 LGTMs obtained

---

internal/setup/setup.go, line 33 at r1 (raw file):

	curr, err := fs.Stat(f.Name)
	if err != nil {
		// TODO: best way to handle this?

At least please add a log line here.

[stephen-soltesz]

Yes, good point regarding binary+config deployments. That was not on my mind when I wrote this.

Reviewable status: complete! 1 of 1 LGTMs obtained

---

internal/setup/setup.go, line 33 at r1 (raw file):

Previously, pboothe (Peter Boothe) wrote…

At least please add a log line here.

Done.

[coveralls]

Pull Request Test Coverage Report for Build 54

• 33 of 33 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+4.5%) to 83.333%

---

Totals
Change from base Build 52:	4.5%
Covered Lines:	130
Relevant Lines:	156

---

💛 - Coveralls