Add option to export metrics to multiple backends

[sk-]

What are you trying to achieve?
We are evaluating different metrics backends, however currently there does not seem to be any way to export the metrics to multiple backends at the same time other than using the OtelCollector.

By paraphrasing open-telemetry/opentelemetry-js#532

this is handy if transitioning from one vendor/OSS project to another for the [metric] backend. You might want to export to both during the transitional phase.

Additional context.

This is currently possible for traces. See for example the documentation added in open-telemetry/opentelemetry-js#537

[joaopgrassi]

Maybe this should be discussed in OpenTelemetry JS? AFAIK you can do this by configuring another exporter/reader on the MeterProvider.

[llc1123]

Calling MeterProvider.addMetricReader multiple times should work?

[sk-]

Cool thanks, attaching multiple readers sounds like the solution I was looking for.