Migrate BatchExporterProcessor to async

src/OpenTelemetry/.publicApi/Experimental/PublicAPI.Unshipped.txt

@@ -24,4 +24,5 @@ static Microsoft.Extensions.Logging.OpenTelemetryLoggingExtensions.UseOpenTeleme
 static Microsoft.Extensions.Logging.OpenTelemetryLoggingExtensions.UseOpenTelemetry(this Microsoft.Extensions.Logging.ILoggingBuilder! builder, System.Action<OpenTelemetry.Logs.LoggerProviderBuilder!>! configure) -> Microsoft.Extensions.Logging.ILoggingBuilder!
 static Microsoft.Extensions.Logging.OpenTelemetryLoggingExtensions.UseOpenTelemetry(this Microsoft.Extensions.Logging.ILoggingBuilder! builder, System.Action<OpenTelemetry.Logs.LoggerProviderBuilder!>? configureBuilder, System.Action<OpenTelemetry.Logs.OpenTelemetryLoggerOptions!>? configureOptions) -> Microsoft.Extensions.Logging.ILoggingBuilder!
 static OpenTelemetry.Sdk.CreateLoggerProviderBuilder() -> OpenTelemetry.Logs.LoggerProviderBuilder!
+virtual OpenTelemetry.BaseExporter<T>.ExportAsync(OpenTelemetry.Batch<T!> batch, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> System.Threading.Tasks.Task<OpenTelemetry.ExportResult>!
 virtual OpenTelemetry.Metrics.FixedSizeExemplarReservoir.OnCollected() -> void

src/OpenTelemetry/BaseExporter.cs

@@ -42,6 +42,17 @@ public abstract class BaseExporter<T> : IDisposable
     /// <returns>Result of the export operation.</returns>
     public abstract ExportResult Export(in Batch<T> batch);
 
+    /// <summary>
+    /// Exports a batch of telemetry objects.
+    /// </summary>
+    /// <param name="batch">Batch of telemetry objects to export.</param>
+    /// <param name="cancellationToken">The cancellation token to cancel the export.</param>
+    /// <returns>Result of the export operation.</returns>
+    public virtual Task<ExportResult> ExportAsync(Batch<T> batch, CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(this.Export(batch));
+    }
+
     /// <summary>
     /// Flushes the exporter, blocks the current thread until flush
     /// completed, shutdown signaled or timed out.