add host metrics (#3277)

* add host metrics

Signed-off-by: yeya24 <yb532204897@gmail.com>

* update hosts readme docs

Signed-off-by: yeya24 <yb532204897@gmail.com>

plugin/hosts/README.md

@@ -72,6 +72,13 @@ hosts [FILE [ZONES...]] {
   is authoritative. If specific zones are listed (for example `in-addr.arpa` and `ip6.arpa`), then only
   queries for those zones will be subject to fallthrough.
 
+## Metrics
+
+If monitoring is enabled (via the *prometheus* directive) then the following metrics are exported:
+
+- `coredns_hosts_entries_count{}` - The combined number of entries in hosts and Corefile.
+- `coredns_hosts_reload_timestamp_seconds{}` - The timestamp of the last reload of hosts file.
+
 ## Examples
 
 Load `/etc/hosts` file.

plugin/hosts/hostsfile.go

@@ -17,6 +17,8 @@ import (
 	"time"
 
 	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
 )
 
 // parseIP calls discards any v6 zone info, before calling net.ParseIP.
@@ -135,6 +137,8 @@ func (h *Hostsfile) readHosts() {
 	h.mtime = stat.ModTime()
 	h.size = stat.Size()
 
+	hostsEntries.WithLabelValues().Set(float64(h.inline.Len() + h.hmap.Len()))
+	hostsReloadTime.Set(float64(stat.ModTime().UnixNano()) / 1e9)
 	h.Unlock()
 }
 
@@ -252,3 +256,19 @@ func (h *Hostsfile) LookupStaticAddr(addr string) []string {
 	copy(hostsCp[len(hosts1):], hosts2)
 	return hostsCp
 }
+
+var (
+	hostsEntries = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "hosts",
+		Name:      "entries_count",
+		Help:      "The combined number of entries in hosts and Corefile.",
+	}, []string{})
+
+	hostsReloadTime = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "hosts",
+		Name:      "reload_timestamp_seconds",
+		Help:      "The timestamp of the last reload of hosts file.",
+	})
+)

plugin/hosts/setup.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 
 	"github.com/caddyserver/caddy"
@@ -57,6 +58,12 @@ func setup(c *caddy.Controller) error {
 		return nil
 	})
 
+	c.OnStartup(func() error {
+		metrics.MustRegister(c, hostsEntries)
+		metrics.MustRegister(c, hostsReloadTime)
+		return nil
+	})
+
 	c.OnShutdown(func() error {
 		close(parseChan)
 		return nil