Added reserve for spans array in BatchSpanProcessor.

    Added reserve for spans array in BatchSpanProcessor.
    
    Helps to allocate the amount of memory needed for number of records so that dynamic memory allocation doesn't happen in the consume method.
    
    .push_back() reallocates memory each time the method is called.
    
    Using .reserve() would avoid memory reallocation as already the memory is allocated.
    
    References:
    https://cplusplus.com/reference/vector/vector/push_back/
    https://cplusplus.com/reference/vector/vector/reserve/

sdk/src/trace/batch_span_processor.cc

@@ -206,6 +206,10 @@ void BatchSpanProcessor::Export()
       NotifyCompletion(notify_force_flush, exporter_, synchronization_data_);
       break;
     }
+
+    //reserves the space for the number of records to be exported
+    spans_arr.reserve(num_records_to_export);
+
     buffer_.Consume(num_records_to_export,
                     [&](CircularBufferRange<AtomicUniquePtr<Recordable>> range) noexcept {
                       range.ForEach([&](AtomicUniquePtr<Recordable> &ptr) {