Implement OpenTelemetry metrics exporter

[unflxw]

We could implement an exporter for OpenTelemetry metrics that calls the extension's metrics functions, similar to the existing OpenTelemetry span exporter. EDIT: We could also use the default OTel HTTP exporter with the existing HTTP metrics -- in fact we probably will want to do that, as a first step towards making the Node.js work the same way as Python.

This would allow us to pick up metrics from the OpenTelemetry instrumentations, and in turn implement magic dashboards for those metrics. Ideally this would allow us to restore the Postgres dashboard that existed in 2.x -- see appsignal/public_config#49 for details.

To do

• Implement OpenTelemetry metrics exporter in Node.js
• Add back metrics previously lost for webvitals and postgres (create separate issues for it)
• Document the metrics support