added span size gauge

Sources/OpenTelemetrySdk/Trace/SpanProcessors/BatchSpanProcessor.swift

@@ -85,9 +85,12 @@ private class BatchWorker: Thread {
     var queue: OperationQueue
 
     private var queueSizeGauge: ObservableLongGauge?
+    private var spanGaugeObserver: ObservableLongGauge?
+
     private var processedSpansCounter: LongCounter
     private let droppedAttrs: [String: AttributeValue]
     private let exportedAttrs: [String: AttributeValue]
+    private let spanGaugeBuilder: LongGaugeBuilder
     init(
         spanExporter: SpanExporter,
         meterProvider: StableMeterProvider,
@@ -123,6 +126,9 @@ private class BatchWorker: Thread {
                 )
             }
 
+        self.spanGaugeBuilder = meter.gaugeBuilder(name: "spanSize")
+            .ofLongs()
+
         processedSpansCounter = meter.counterBuilder(name: "processedSpans")
             .setUnit("1")
             .setDescription("The number of spans processed by the BatchSpanProcessor. [dropped=true if they were dropped due to high throughput]")
@@ -137,6 +143,12 @@ private class BatchWorker: Thread {
         ]
     }
 
+    deinit {
+        // Cleanup all gauge observer
+        self.queueSizeGauge?.close()
+        self.spanGaugeObserver?.close()
+    }
+
   func addSpan(span: ReadableSpan) {
     cond.lock()
     defer { cond.unlock() }
@@ -145,8 +157,21 @@ private class BatchWorker: Thread {
         processedSpansCounter.add(value: 1, attribute: droppedAttrs)
       return
     }
-    // TODO: Record a gauge for referenced spans.
     spanList.append(span)
+
+      // If there is any observer before, let's close it
+      self.spanGaugeObserver?.close()
+      // Subscribe to new gauge observer
+      self.spanGaugeObserver = self.spanGaugeBuilder
+          .buildWithCallback { [count = spanList.count] result in
+              result.record(
+                  value: count,
+                  attributes: [
+                      BatchSpanProcessor.SPAN_PROCESSOR_TYPE_LABEL: .string(BatchSpanProcessor.SPAN_PROCESSOR_TYPE_VALUE)
+                  ]
+              )
+          }
+
     // Notify the worker thread that at half of the queue is available. It will take
     // time anyway for the thread to wake up.
     if spanList.count >= halfMaxQueueSize {