Optimize evictedQueue implementation and use (#2556)

* Optimize evictedQueue impl and use

Avoid unnecessary allocations in the recordingSpan by using an
evictedQueue type instead of a pointer to one.

Lazy allocate the evictedQueue queue to prevent unnecessary operations
for spans without any use of the queue.

Document the evictedQueue

* Fix grammar

sdk/trace/evictedqueue.go

@@ -14,24 +14,25 @@
 
 package trace // import "go.opentelemetry.io/otel/sdk/trace"
 
+// evictedQueue is a FIFO queue with a configurable capacity.
 type evictedQueue struct {
 	queue        []interface{}
 	capacity     int
 	droppedCount int
 }
 
-func newEvictedQueue(capacity int) *evictedQueue {
-	eq := &evictedQueue{
-		capacity: capacity,
-		queue:    make([]interface{}, 0),
-	}
-
-	return eq
+func newEvictedQueue(capacity int) evictedQueue {
+	// Do not pre-allocate queue, do this lazily.
+	return evictedQueue{capacity: capacity}
 }
 
+// add adds value to the evictedQueue eq. If eq is at capacity, the oldest
+// queued value will be discarded and the drop count incremented.
 func (eq *evictedQueue) add(value interface{}) {
 	if len(eq.queue) == eq.capacity {
-		eq.queue = eq.queue[1:]
+		// Drop first-in while avoiding allocating more capacity to eq.queue.
+		copy(eq.queue[:eq.capacity-1], eq.queue[1:])
+		eq.queue = eq.queue[:eq.capacity-1]
 		eq.droppedCount++
 	}
 	eq.queue = append(eq.queue, value)

sdk/trace/span.go

@@ -142,10 +142,10 @@ type recordingSpan struct {
 	attributes *attributesMap
 
 	// events are stored in FIFO queue capped by configured limit.
-	events *evictedQueue
+	events evictedQueue
 
 	// links are stored in FIFO queue capped by configured limit.
-	links *evictedQueue
+	links evictedQueue
 
 	// executionTracerTaskEnd ends the execution tracer span.
 	executionTracerTaskEnd func()