include build info in Prometheus metrics (#22819)

Related to: https://github.com/go-gitea/gitea/issues/18061

This PR adds build info to the Prometheus metrics. This includes:
- goarch: https://pkg.go.dev/runtime#GOARCH
- goos: https://pkg.go.dev/runtime#pkg-constants
- goversion: https://pkg.go.dev/runtime#Version
- gitea version: just exposes the existing
code.gitea.io/gitea/modules/setting.AppVer

It's a similar approach to what some other Golang projects are doing,
e.g. Prometheus:
https://github.com/prometheus/common/blob/main/version/info.go

example /metrics response from Prometheus:
```
# HELP prometheus_build_info A metric with a constant '1' value labeled by version, revision, branch, goversion from which prometheus was built, and the goos and goarch for the build.
# TYPE prometheus_build_info gauge
prometheus_build_info{branch="HEAD",goarch="amd64",goos="linux",goversion="go1.19.4",revision="c0d8a56c69014279464c0e15d8bfb0e153af0dab",version="2.41.0"} 1
```

/metrics response from gitea with this PR:
```
# HELP gitea_build_info Build information
# TYPE gitea_build_info gauge
gitea_build_info{goarch="amd64",goos="linux",goversion="go1.20",version="2c6cc0b8c"} 1
```

Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

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Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

modules/metrics/collector.go

@@ -4,7 +4,10 @@
 package metrics
 
 import (
+	"runtime"
+
 	activities_model "code.gitea.io/gitea/models/activities"
+	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/prometheus/client_golang/prometheus"
 )
@@ -17,6 +20,7 @@ type Collector struct {
 	Accesses           *prometheus.Desc
 	Actions            *prometheus.Desc
 	Attachments        *prometheus.Desc
+	BuildInfo          *prometheus.Desc
 	Comments           *prometheus.Desc
 	Follows            *prometheus.Desc
 	HookTasks          *prometheus.Desc
@@ -62,6 +66,16 @@ func NewCollector() Collector {
 			"Number of Attachments",
 			nil, nil,
 		),
+		BuildInfo: prometheus.NewDesc(
+			namespace+"build_info",
+			"Build information",
+			[]string{
+				"goarch",
+				"goos",
+				"goversion",
+				"version",
+			}, nil,
+		),
 		Comments: prometheus.NewDesc(
 			namespace+"comments",
 			"Number of Comments",
@@ -195,6 +209,7 @@ func (c Collector) Describe(ch chan<- *prometheus.Desc) {
 	ch <- c.Accesses
 	ch <- c.Actions
 	ch <- c.Attachments
+	ch <- c.BuildInfo
 	ch <- c.Comments
 	ch <- c.Follows
 	ch <- c.HookTasks
@@ -241,6 +256,15 @@ func (c Collector) Collect(ch chan<- prometheus.Metric) {
 		prometheus.GaugeValue,
 		float64(stats.Counter.Attachment),
 	)
+	ch <- prometheus.MustNewConstMetric(
+		c.BuildInfo,
+		prometheus.GaugeValue,
+		1,
+		runtime.GOARCH,
+		runtime.GOOS,
+		runtime.Version(),
+		setting.AppVer,
+	)
 	ch <- prometheus.MustNewConstMetric(
 		c.Comments,
 		prometheus.GaugeValue,