perf(export): do not use splice if fit in one batch

packages/opentelemetry-sdk-trace-base/src/export/BatchSpanProcessorBase.ts

@@ -34,8 +34,7 @@ import { SpanExporter } from './SpanExporter';
  * the SDK then pushes them to the exporter pipeline.
  */
 export abstract class BatchSpanProcessorBase<T extends BufferConfig>
-  implements SpanProcessor
-{
+  implements SpanProcessor {
   private readonly _maxExportBatchSize: number;
   private readonly _maxQueueSize: number;
   private readonly _scheduledDelayMillis: number;
@@ -87,7 +86,7 @@ export abstract class BatchSpanProcessorBase<T extends BufferConfig>
   }
 
   // does nothing.
-  onStart(_span: Span, _parentContext: Context): void {}
+  onStart(_span: Span, _parentContext: Context): void { }
 
   onEnd(span: ReadableSpan): void {
     if (this._shutdownOnce.isCalled) {
@@ -181,7 +180,14 @@ export abstract class BatchSpanProcessorBase<T extends BufferConfig>
         // Reset the finished spans buffer here because the next invocations of the _flush method
         // could pass the same finished spans to the exporter if the buffer is cleared
         // outside the execution of this callback.
-        const spans = this._finishedSpans.splice(0, this._maxExportBatchSize);
+        let spans: ReadableSpan[];
+        if (this._finishedSpans.length <= this._maxExportBatchSize) {
+          spans = this._finishedSpans;
+          this._finishedSpans = [];
+        }
+        else {
+          spans = this._finishedSpans.splice(0, this._maxExportBatchSize);
+        }
 
         const doExport = () =>
           this._exporter.export(spans, result => {
@@ -191,7 +197,7 @@ export abstract class BatchSpanProcessorBase<T extends BufferConfig>
             } else {
               reject(
                 result.error ??
-                  new Error('BatchSpanProcessor: span export failed')
+                new Error('BatchSpanProcessor: span export failed')
               );
             }
           });