plugin/template (#1298)

* Add a template plugin

The template plugin matches the incoming query by class, type and regex
and templates a response with go templates.

* Fix go style errors

* Fix template README example

* Fix corefile example in plugin/template

* Clarify plugin/template/README.md

Add more details and external links where needed.

* Fix code issues in plugin/template

* Add template metrics

* Add section and template to template plugin metrics

* Fix style / remove extra newline on go imports

* Fix typo in plugin/template/README.md

* Update README.md

I've change the format a bit in a PR that I merged yesterday.

* Add authority section to plugin/template

* Fix naming of incoming query name in plugin/template/README.md

* Fix doc syntax in plugin/template/README.md

* Add authority section to plugin/template/README.md config overview

* Add metric labels to plugin/template/README.md metrics section

* Use request.Request to pass state to the template matcher

plugin/template/metrics.go

@@ -0,0 +1,43 @@
+package template
+
+import (
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics for template.
+var (
+	TemplateMatchesCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "template",
+		Name:      "matches_total",
+		Help:      "Counter of template regex matches.",
+	}, []string{"regex"})
+	TemplateFailureCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "template",
+		Name:      "template_failures_total",
+		Help:      "Counter of go template failures.",
+	}, []string{"regex", "section", "template"})
+	TemplateRRFailureCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "template",
+		Name:      "rr_failures_total",
+		Help:      "Counter of mis-templated RRs.",
+	}, []string{"regex", "section", "template"})
+)
+
+// OnStartupMetrics sets up the metrics on startup.
+func OnStartupMetrics() error {
+	metricsOnce.Do(func() {
+		prometheus.MustRegister(TemplateMatchesCount)
+		prometheus.MustRegister(TemplateFailureCount)
+		prometheus.MustRegister(TemplateRRFailureCount)
+	})
+	return nil
+}
+
+var metricsOnce sync.Once