feat: new Prometheus metrics build_info

[nlamirault]

Overview

Add a new metric dex_build_info, with version which will be set to the Git tag version.

$ curl -s http://127.0.0.1:5558/metrics | grep build
# HELP dex_build_info A metric with a constant '1' value labeled by version from which Dex was built.
# TYPE dex_build_info gauge
dex_build_info{goarch="arm64",goversion="go1.22.2",version="43a9a144a710803b6a3036368cec3bec9f6f02de-dirty"} 1 

What this PR does / why we need it

Special notes for your reviewer

[nabokihms]

Some suggestion to make the metrics match Kubernetes build info labels:

1. go_version instead of goversion (for readability)
2. platform instead ofgoarch, because this label is usually used to understand where you can run the software

Overall, PR looks great! Thanks for your effort.

[nabokihms]

Suggested change

	[]string{"version", "goversion", "goarch"},
	[]string{"version", "go_version", "platform"},

[nabokihms]

@nlamirault could you please fix the lint error?

[nlamirault]

I don't know how to fix that:

└─ ➜ go fmt cmd/dex/serve.go

└─ ➜ make lint
golangci-lint has version 1.59.1 built with go1.22.4 from 1a55854 on 2024-06-08T22:05:49Z
WARN [config_reader] The configuration option `run.skip-dirs` is deprecated, please use `issues.exclude-dirs`.
WARN [config_reader] The configuration option `linters.gci.local-prefixes` is deprecated, please use `prefix()` inside `linters.gci.sections`.
cmd/dex/serve.go:54: File is not `gofumpt`-ed (gofumpt)
var (
        buildInfo = prometheus.NewGaugeVec(
                prometheus.GaugeOpts{
                        Name:      "build_info",
                        Namespace: "dex",
                        Help:      "A metric with a constant '1' value labeled by version from which Dex was built.",
                },
                []string{"version", "go_version", "platform"},
        )
make: *** [lint] Error 1

[nabokihms]

It should help:

make help
make fix