Fixes dynamic registration of views bug (#456)

Prometheus's default registry appears to block registration of any other
metrics based on there being >= 1 duplicate in the set of metrics returned
from Describe to register. While Describe's API seems to indicate that a
collector should return a superset of metrics, and while this makes perfect
sense with the Unregister logic, it does not seem to play nicely with us
running register on the same collector repeatedly with different metrics, nor
with calling unregister and then attempting to register everything once again.
that is to say, Prometheus client seems to explicitly disallow the thing we're
seemingly trying to do. upstream: prometheus/client_golang#47

This patch will:
* No longer call Unregister and then Register, as this is a case handled by
prometheus: https://github.com/prometheus/client_golang/blob/fcc130e101e76c5d303513d0e28f4b6d732845c7/prometheus/registry.go#L112-L117
* Only return metrics that are not yet registered to Prometheus registry

exporter/prometheus/prometheus.go

@@ -92,9 +92,9 @@ func (c *collector) registerViews(views ...*view.View) {
 	count := 0
 	for _, view := range views {
 		sig := viewSignature(c.opts.Namespace, view)
-		c.registeredViewsMu.Lock()
+		c.viewsMu.Lock()
 		_, ok := c.registeredViews[sig]
-		c.registeredViewsMu.Unlock()
+		c.viewsMu.Unlock()
 
 		if !ok {
 			desc := prometheus.NewDesc(
@@ -103,17 +103,16 @@ func (c *collector) registerViews(views ...*view.View) {
 				tagKeysToLabels(view.TagKeys()),
 				nil,
 			)
-			c.registeredViewsMu.Lock()
-			c.registeredViews[sig] = desc
-			c.registeredViewsMu.Unlock()
+			c.viewsMu.Lock()
+			c.newViews[sig] = desc
+			c.viewsMu.Unlock()
 			count++
 		}
 	}
 	if count == 0 {
 		return
 	}
 
-	c.reg.Unregister(c)
 	if err := c.reg.Register(c); err != nil {
 		c.opts.onError(fmt.Errorf("cannot register the collector: %v", err))
 	}
@@ -160,9 +159,14 @@ type collector struct {
 	// Collect is invoked and the cycle is repeated.
 	viewData map[string]*view.Data
 
-	registeredViewsMu sync.Mutex
+	// protects registeredViews and newViews
+	viewsMu sync.Mutex
 	// registeredViews maps a view to a prometheus desc.
 	registeredViews map[string]*prometheus.Desc
+
+	// newViews is the set of views that are
+	// queued to be registered with Prometheus.
+	newViews map[string]*prometheus.Desc
 }
 
 func (c *collector) addViewData(vd *view.Data) {
@@ -175,12 +179,21 @@ func (c *collector) addViewData(vd *view.Data) {
 }
 
 func (c *collector) Describe(ch chan<- *prometheus.Desc) {
-	c.registeredViewsMu.Lock()
-	registered := make(map[string]*prometheus.Desc)
-	for k, desc := range c.registeredViews {
-		registered[k] = desc
+	// NOTE that this will not work properly if calling
+	// prometheus.Registerer.Unregister, but the semantics around prometheus client
+	// do not seem to lend to us being able to register all our views repeatedly.
+	// this does not return the super set of desc, just the subset that we have not
+	// yet registered with prometheus (it yells about dupes, and explicitly
+	// disallows unregistering all metrics and then registering them again).
+
+	c.viewsMu.Lock()
+	registered := make([]*prometheus.Desc, 0, len(c.newViews))
+	for k, desc := range c.newViews {
+		registered = append(registered, desc)
+		c.registeredViews[k] = desc
+		delete(c.newViews, k)
 	}
-	c.registeredViewsMu.Unlock()
+	c.viewsMu.Unlock()
 
 	for _, desc := range registered {
 		ch <- desc
@@ -197,9 +210,9 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 
 	for _, vd := range viewData {
 		sig := viewSignature(c.opts.Namespace, vd.View)
-		c.registeredViewsMu.Lock()
+		c.viewsMu.Lock()
 		desc := c.registeredViews[sig]
-		c.registeredViewsMu.Unlock()
+		c.viewsMu.Unlock()
 
 		for _, row := range vd.Rows {
 			metric, err := c.toMetric(desc, vd.View, row)
@@ -260,6 +273,7 @@ func newCollector(opts Options, registrar *prometheus.Registry) *collector {
 		reg:             registrar,
 		opts:            opts,
 		registeredViews: make(map[string]*prometheus.Desc),
+		newViews:        make(map[string]*prometheus.Desc),
 		viewData:        make(map[string]*view.Data),
 	}
 }