feat: coredns_forge_info{version} metric

this adds metric with current p2p-forge build version,
useful for tracking when which box got updated to what

docs/METRICS.md

@@ -6,6 +6,8 @@ It includes default [Prometheus Glient metrics](https://prometheus.io/docs/guide
 
 ### Forge metrics
 
+- `coredns_forge_info{version}` - info about p2p-forge instance, useful for tracking when version update occured on each box
+
 #### `ipparser` plugin (DNS A/AAAA)
 
 - `coredns_forge_ipparser_responses_total{type}` - dynamic DNS `A`/`AAAA` responses generated by `ipparser` plugin for `ip.peerid.domain` and `peerid.domain`, including `NODATA-*` ones.

main.go

@@ -5,6 +5,7 @@ import (
 
 	// Load CoreDNS + p2p-forge plugins
 	_ "github.com/ipshipyard/p2p-forge/plugins"
+	"github.com/prometheus/client_golang/prometheus"
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/coremain"
@@ -39,9 +40,22 @@ func init() {
 
 func main() {
 	fmt.Printf("%s %s\n", name, version) // always print version
+	registerVersionMetric()
 	err := godotenv.Load()
 	if err == nil {
 		fmt.Println(".env found and loaded")
 	}
 	coremain.Run()
 }
+
+func registerVersionMetric() {
+	m := prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace:   "coredns",
+		Subsystem:   "forge",
+		Name:        "info",
+		Help:        "Information about p2p-forge instance.",
+		ConstLabels: prometheus.Labels{"version": version},
+	})
+	prometheus.MustRegister(m)
+	m.Set(1)
+}