Ensure Re-register of metrics variables after a reload (#2080)

* - ensure plugins that use prometheus.MustRegister, re-register after reload
- removing once.Do on the startup function was simplest way to do it.

* - fix underscored names (advice of bot)

* - tune existing UT for reload, and add a test verifying failing reload does not prevent correct registering for metrics

* - ensure different ports for tests that can run in same time ..

plugin/autopath/metrics.go

@@ -1,8 +1,6 @@
 package autopath
 
 import (
-	"sync"
-
 	"github.com/coredns/coredns/plugin"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -16,5 +14,3 @@ var (
 		Help:      "Counter of requests that did autopath.",
 	}, []string{"server"})
 )
-
-var once sync.Once

plugin/autopath/setup.go

@@ -26,7 +26,7 @@ func setup(c *caddy.Controller) error {
 	}
 
 	c.OnStartup(func() error {
-		once.Do(func() { metrics.MustRegister(c, autoPathCount) })
+		metrics.MustRegister(c, autoPathCount)
 		return nil
 	})
 

plugin/cache/handler.go

@@ -3,7 +3,6 @@ package cache
 import (
 	"context"
 	"math"
-	"sync"
 	"time"
 
 	"github.com/coredns/coredns/plugin"
@@ -126,5 +125,3 @@ var (
 		Help:      "The number responses that are not cached, because the reply is malformed.",
 	}, []string{"server"})
 )
-
-var once sync.Once

plugin/cache/setup.go

@@ -34,11 +34,9 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			metrics.MustRegister(c,
-				cacheSize, cacheHits, cacheMisses,
-				cachePrefetches, cacheDrops)
-		})
+		metrics.MustRegister(c,
+			cacheSize, cacheHits, cacheMisses,
+			cachePrefetches, cacheDrops)
 		return nil
 	})
 

plugin/dnssec/handler.go

@@ -2,7 +2,6 @@ package dnssec
 
 import (
 	"context"
-	"sync"
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics"
@@ -81,5 +80,3 @@ var (
 
 // Name implements the Handler interface.
 func (d Dnssec) Name() string { return "dnssec" }
-
-var once sync.Once

plugin/dnssec/setup.go

@@ -35,9 +35,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			metrics.MustRegister(c, cacheSize, cacheHits, cacheMisses)
-		})
+		metrics.MustRegister(c, cacheSize, cacheHits, cacheMisses)
 		return nil
 	})
 

plugin/forward/metrics.go

@@ -1,8 +1,6 @@
 package forward
 
 import (
-	"sync"
-
 	"github.com/coredns/coredns/plugin"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -48,5 +46,3 @@ var (
 		Help:      "Gauge of open sockets per upstream.",
 	}, []string{"to"})
 )
-
-var once sync.Once

plugin/forward/setup.go

@@ -38,9 +38,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() {
-			metrics.MustRegister(c, RequestCount, RcodeCount, RequestDuration, HealthcheckFailureCount, SocketGauge)
-		})
+		metrics.MustRegister(c, RequestCount, RcodeCount, RequestDuration, HealthcheckFailureCount, SocketGauge)
 		return f.OnStartup()
 	})
 

plugin/health/setup.go

@@ -55,7 +55,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() { metrics.MustRegister(c, HealthDuration) })
+		metrics.MustRegister(c, HealthDuration)
 		return nil
 	})
 

plugin/proxy/metrics.go

@@ -1,8 +1,6 @@
 package proxy
 
 import (
-	"sync"
-
 	"github.com/coredns/coredns/plugin"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -36,5 +34,3 @@ func familyToString(f int) string {
 	}
 	return ""
 }
-
-var once sync.Once

plugin/proxy/setup.go

@@ -33,7 +33,7 @@ func setup(c *caddy.Controller) error {
 	})
 
 	c.OnStartup(func() error {
-		once.Do(func() { metrics.MustRegister(c, RequestCount, RequestDuration) })
+		metrics.MustRegister(c, RequestCount, RequestDuration)
 		return nil
 	})
 

plugin/template/metrics.go

@@ -1,8 +1,6 @@
 package template
 
 import (
-	"sync"
-
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/metrics"
 
@@ -34,13 +32,9 @@ var (
 // OnStartupMetrics sets up the metrics on startup.
 func setupMetrics(c *caddy.Controller) error {
 	c.OnStartup(func() error {
-		metricsOnce.Do(func() {
-			metrics.MustRegister(c, templateMatchesCount, templateFailureCount, templateRRFailureCount)
-		})
+		metrics.MustRegister(c, templateMatchesCount, templateFailureCount, templateRRFailureCount)
 		return nil
 	})
 
 	return nil
 }
-
-var metricsOnce sync.Once