chore: Update MetricsExporter class in example

Now, there's an extra Metrics:: namespace

examples/metrics_sdk/metrics_collect_otlp.rb

@@ -20,7 +20,7 @@
 
 OpenTelemetry::SDK.configure
 
-otlp_metric_exporter = OpenTelemetry::Exporter::OTLP::MetricsExporter.new
+otlp_metric_exporter = OpenTelemetry::Exporter::OTLP::Metrics::MetricsExporter.new
 
 OpenTelemetry.meter_provider.add_metric_reader(otlp_metric_exporter)