feat: Support OTEL metrics

### Description

Add a OTel metrics support so that the OpenAPM emits metrics in OTLP format and directly pushes it to a OTLP compatible endpoint like `vmagent` or `Otel collector`.

OpenAPM should have a mode option and should have two modes namely `openmetrics` (default) and `opentelemetry`.

```js
const openapm = new OpenAPM({
    mode: 'opentelemetry'
})
```
