Improve JS Instrumentation Docs (#4052)

Author: legalimpurity

content/en/docs/languages/js/instrumentation.md

@@ -1261,10 +1261,9 @@ const metricReader = new PeriodicExportingMetricReader({
 
 const myServiceMeterProvider = new MeterProvider({
   resource: resource,
+  readers: [metricReader],
 });
 
-myServiceMeterProvider.addMetricReader(metricReader);
-
 // Set this MeterProvider to be global to the app being instrumented.
 opentelemetry.metrics.setGlobalMeterProvider(myServiceMeterProvider);
 ```
@@ -1299,10 +1298,9 @@ const metricReader = new PeriodicExportingMetricReader({
 
 const myServiceMeterProvider = new MeterProvider({
   resource: resource,
+  readers: [metricReader],
 });
 
-myServiceMeterProvider.addMetricReader(metricReader);
-
 // Set this MeterProvider to be global to the app being instrumented.
 opentelemetry.metrics.setGlobalMeterProvider(myServiceMeterProvider);
 ```