docs(readme): exporter should be metricExporter (#3688)

Author: hongbo-miao

experimental/packages/opentelemetry-exporter-metrics-otlp-http/README.md

@@ -58,7 +58,7 @@ const collectorOptions = {
   url: '<opentelemetry-collector-url>', // url is optional and can be omitted - default is http://localhost:4318/v1/metrics
   concurrencyLimit: 1, // an optional limit on pending requests
 };
-const exporter = new OTLPMetricExporter(collectorOptions);
+const metricExporter = new OTLPMetricExporter(collectorOptions);
 const meterProvider = new MeterProvider({});
 
 meterProvider.addMetricReader(new PeriodicExportingMetricReader({