plugin/metrics: add MustRegister function (#1648)

This registers the Collectors iff the metrics plugin has been loaded.
Safes a bunch of code in each and every plugin's setup code.

9 files changed, 34 insertions, 72 deletions

diff --git a/plugin/autopath/setup.go b/plugin/autopath/setup.go
index ac0f05ec5..6015c7460 100644
--- a/plugin/autopath/setup.go
+++ b/plugin/autopath/setup.go
@@ -26,15 +26,7 @@ func setup(c *caddy.Controller) error {
 	}
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(AutoPathCount)
-			}
-		})
+		once.Do(func() { metrics.MustRegister(c, AutoPathCount) })
 		return nil
 	})
 
diff --git a/plugin/cache/setup.go b/plugin/cache/setup.go
index 57edd3246..8fe8eb59c 100644
--- a/plugin/cache/setup.go
+++ b/plugin/cache/setup.go
@@ -32,18 +32,10 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(func() error {
 		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(cacheSize)
-				x.MustRegister(cacheCapacity)
-				x.MustRegister(cacheHits)
-				x.MustRegister(cacheMisses)
-				x.MustRegister(cachePrefetches)
-				x.MustRegister(cacheDrops)
-			}
+			metrics.MustRegister(c,
+				cacheSize, cacheCapacity,
+				cacheHits, cacheMisses,
+				cachePrefetches, cacheDrops)
 		})
 		return nil
 	})
diff --git a/plugin/dnssec/setup.go b/plugin/dnssec/setup.go
index efcc1aa85..2f82315f8 100644
--- a/plugin/dnssec/setup.go
+++ b/plugin/dnssec/setup.go
@@ -33,16 +33,7 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(func() error {
 		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(cacheSize)
-				x.MustRegister(cacheCapacity)
-				x.MustRegister(cacheHits)
-				x.MustRegister(cacheMisses)
-			}
+			metrics.MustRegister(c, cacheSize, cacheCapacity, cacheHits, cacheMisses)
 		})
 		return nil
 	})
diff --git a/plugin/forward/setup.go b/plugin/forward/setup.go
index 71a5ec1f0..137137d45 100644
--- a/plugin/forward/setup.go
+++ b/plugin/forward/setup.go
@@ -38,17 +38,7 @@ func setup(c *caddy.Controller) error {
 
 	c.OnStartup(func() error {
 		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(RequestCount)
-				x.MustRegister(RcodeCount)
-				x.MustRegister(RequestDuration)
-				x.MustRegister(HealthcheckFailureCount)
-				x.MustRegister(SocketGauge)
-			}
+			metrics.MustRegister(c, RequestCount, RcodeCount, RequestDuration, HealthcheckFailureCount, SocketGauge)
 		})
 		return f.OnStartup()
 	})
diff --git a/plugin/health/overloaded.go b/plugin/health/overloaded.go
index 976cafa84..06b0b65e1 100644
--- a/plugin/health/overloaded.go
+++ b/plugin/health/overloaded.go
@@ -49,4 +49,4 @@ var (
 	})
 )
 
-var onceMetric sync.Once
+var once sync.Once
diff --git a/plugin/health/setup.go b/plugin/health/setup.go
index acb194a34..bd5b5c3fc 100644
--- a/plugin/health/setup.go
+++ b/plugin/health/setup.go
@@ -55,15 +55,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		onceMetric.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(HealthDuration)
-			}
-		})
+		once.Do(func() { metrics.MustRegister(c, HealthDuration) })
 		return nil
 	})
 
diff --git a/plugin/metrics/register.go b/plugin/metrics/register.go
new file mode 100644
index 000000000..bbc388d0c
--- /dev/null
+++ b/plugin/metrics/register.go
@@ -0,0 +1,23 @@
+package metrics
+
+import (
+	"github.com/coredns/coredns/core/dnsserver"
+
+	"github.com/mholt/caddy"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// MustRegister registers the prometheus Collectors when the metrics middleware is used.
+func MustRegister(c *caddy.Controller, cs ...prometheus.Collector) {
+	m := dnsserver.GetConfig(c).Handler("prometheus")
+	if m == nil {
+		return
+	}
+	x, ok := m.(*Metrics)
+	if !ok {
+		return
+	}
+	for _, c := range cs {
+		x.MustRegister(c)
+	}
+}
diff --git a/plugin/proxy/setup.go b/plugin/proxy/setup.go
index eff7c63f4..268579751 100644
--- a/plugin/proxy/setup.go
+++ b/plugin/proxy/setup.go
@@ -30,16 +30,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(RequestCount)
-				x.MustRegister(RequestDuration)
-			}
-		})
+		once.Do(func() { metrics.MustRegister(c, RequestCount, RequestDuration) })
 		return nil
 	})
 
diff --git a/plugin/template/metrics.go b/plugin/template/metrics.go
index 4cdaa5321..bc2594782 100644
--- a/plugin/template/metrics.go
+++ b/plugin/template/metrics.go
@@ -3,7 +3,6 @@ package template
 import (
 	"sync"
 
-	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics"
 
@@ -37,15 +36,7 @@ var (
 func setupMetrics(c *caddy.Controller) error {
 	c.OnStartup(func() error {
 		metricsOnce.Do(func() {
-			m := dnsserver.GetConfig(c).Handler("prometheus")
-			if m == nil {
-				return
-			}
-			if x, ok := m.(*metrics.Metrics); ok {
-				x.MustRegister(TemplateMatchesCount)
-				x.MustRegister(TemplateFailureCount)
-				x.MustRegister(TemplateRRFailureCount)
-			}
+			metrics.MustRegister(c, TemplateMatchesCount, TemplateFailureCount, TemplateRRFailureCount)
 		})
 		return nil
 	})