prometheus: Implement basic Prometheus support

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

lib/main.go

@@ -30,6 +30,7 @@ import (
 	"github.com/purpleidea/mgmt/gapi"
 	"github.com/purpleidea/mgmt/pgp"
 	"github.com/purpleidea/mgmt/pgraph"
+	"github.com/purpleidea/mgmt/prometheus"
 	"github.com/purpleidea/mgmt/recwatch"
 	"github.com/purpleidea/mgmt/remote"
 	"github.com/purpleidea/mgmt/resources"
@@ -93,6 +94,9 @@ type Main struct {
 	PgpIdentity *string
 	pgpKeys     *pgp.PGP // agent key pair
 
+	Prometheus       bool   // enable prometheus metrics
+	PrometheusListen string // prometheus instance bind specification
+
 	exit chan error // exit signal
 }
 
@@ -223,6 +227,21 @@ func (obj *Main) Run() error {
 		return errwrap.Wrapf(err, "Can't create pgraph prefix")
 	}
 
+	var prom *prometheus.Prometheus
+	if obj.Prometheus {
+		prom = &prometheus.Prometheus{
+			Listen: obj.PrometheusListen,
+		}
+		if err := prom.Init(); err != nil {
+			return errwrap.Wrapf(err, "Can't create initiate Prometheus instance")
+		}
+
+		log.Printf("Main: Prometheus: Starting instance on %s", prom.Listen)
+		if err := prom.Start(); err != nil {
+			return errwrap.Wrapf(err, "Can't start initiate Prometheus instance")
+		}
+	}
+
 	if !obj.NoPgp {
 		pgpPrefix := fmt.Sprintf("%s/", path.Join(prefix, "pgp"))
 		if err := os.MkdirAll(pgpPrefix, 0770); err != nil {
@@ -552,6 +571,14 @@ func (obj *Main) Run() error {
 		reterr = multierr.Append(reterr, err) // list of errors
 	}
 
+	if obj.Prometheus {
+		log.Printf("Main: Prometheus: Stopping instance")
+		if err := prom.Stop(); err != nil {
+			err = errwrap.Wrapf(err, "Prometheus instance exited poorly!")
+			reterr = multierr.Append(reterr, err)
+		}
+	}
+
 	if obj.Flags.Debug {
 		log.Printf("Main: Graph: %v", G)
 	}