Add build flags

[pgporada]

Added a new //util package which contains some variables set by the builder at build time to output more information during process startup. This was largely copied from boulder. See below:

$ ./unbound_exporter 
level=info ts=2024-05-20T15:44:53.420Z caller=unbound_exporter.go:553 msg="Starting unbound_exporter" version="(version=go1.22.2, branch=main, revision=ab17c00)"

Fixes #75

[aarongable]

LGTM, just some naming a documentation-consistency changes.

[aarongable]

Suggested change

	run: go build -ldflags "-X github.com/letsencrypt/unbound_exporter/util.BuildID=$(git rev-parse --short HEAD) -X github.com/letsencrypt/unbound_exporter/util.BuildBranch=$(git rev-parse --abbrev-ref HEAD) -X github.com/letsencrypt/unbound_exporter/util.BuildTime=$(date +%F-%T -u)"
	run: go build -ldflags "-X github.com/letsencrypt/unbound_exporter/build.Revision=$(git rev-parse --short HEAD) -X github.com/letsencrypt/unbound_exporter/build.Branch=$(git rev-parse --abbrev-ref HEAD)"

[aarongable]

Suggested change

	// BuildID is set by the compiler (using -ldflags "-X util.BuildID $(git rev-parse --short HEAD)")
	// and is used by GetID
	var BuildID string
	// Revision is set by the compiler (using -ldflags "-X build.Revision $(git rev-parse --short HEAD)")
	// and is used by GetRevision
	var Revision string

[aarongable]

Suggested change

	// BuildBranch is set by the compiler and is used by GetBranch
	var BuildBranch string
	// Branch is set by the compiler and is used by GetBranch
	var Branch string

[aarongable]

Suggested change

	// GetBuildID identifies what build is running.
	func GetID() (BuildID string) {
	if BuildID != "" {
	return BuildID
	// GetRevision identifies what commit this binary was built from.
	func GetRevision() string {
	if Revision != "" {
	return Revision

[aarongable]

Suggested change

	// BuildBranch is set by the compiler and is used by GetBranch
	// BuildBranch is set by the compiler (using -ldflags "-X build.Branch $git rev-parse --abbrev-rev HEAD)")
	// and is used by GetBranch

[aarongable]

Suggested change

	// BuildID is set by the compiler (using -ldflags "-X util.BuildID $(git rev-parse --short HEAD)")
	// Revision is set by the compiler (using -ldflags "-X build.Revision $(git rev-parse --short HEAD)")

[aarongable]

Suggested change

	// GetBranch identifies the building host
	func GetBranch() (BuildBranch string) {
	if BuildBranch != "" {
	return BuildBranch
	// GetBranch identifies what git branch this binary was built from.
	func GetBranch() string {
	if Branch != "" {
	return Branch

[jpds]

I was planning to push up a PR which would add a unbound_exporter_build_info metric once #75 landed.

This is what the other Prometheus exporters do: https://github.com/prometheus/node_exporter/blob/3afc0a341e3c2c6605b18a882fb045318c18c444/node_exporter.go#L123 - the functionality uses the prometheus.Version data.