add docappender.blocked.add counter

This monotonic counter increments every time Appender.Add
detects that bulkItems channel length and capacity are
equal before sending the created BulkIndexerItem to the
channel.

As bulkItems is a buffered channel, monitoring length and
capacity allow us to detect possible blocking events when
sending to the channel due to exhausted capacity.

As Appender.Add could be called from multiple goroutines,
we are not guaranteed that every time this metric is
incremented the channel send will block, but should give us
a reasonably approximated picture.

appender.go

@@ -75,6 +75,7 @@ type Appender struct {
 	availableBulkRequests  int64
 	activeCreated          int64
 	activeDestroyed        int64
+	blockedAdd             int64
 
 	scalingInfo atomic.Value
 
@@ -278,6 +279,10 @@ func (a *Appender) Add(ctx context.Context, index string, document io.WriterTo)
 		Index: index,
 		Body:  document,
 	}
+	if len(a.bulkItems) == cap(a.bulkItems) {
+		a.addCount(1, &a.blockedAdd, a.metrics.blockedAdd)
+	}
+
 	select {
 	case <-ctx.Done():
 		return ctx.Err()

metric.go

@@ -38,6 +38,7 @@ type metrics struct {
 	availableBulkRequests  metric.Int64UpDownCounter
 	activeCreated          metric.Int64Counter
 	activeDestroyed        metric.Int64Counter
+	blockedAdd             metric.Int64Counter
 }
 
 type histogramMetric struct {
@@ -132,6 +133,11 @@ func newMetrics(cfg Config) (metrics, error) {
 			description: "The number of times an active indexer was destroyed.",
 			p:           &ms.activeDestroyed,
 		},
+		{
+			name:        "docappender.blocked.add",
+			description: "The number of times Add could block due to exhausted capacity in bulkItems channel",
+			p:           &ms.blockedAdd,
+		},
 	}
 	for _, m := range counters {
 		err := newInt64Counter(meter, m)