Allow additional TracingInstrumentations without re-implementing getIgnoreUrls() or faro.initialize()

[pshahid]

Description

When attempting to insert additional TracingInstrumentations other than the default OTEL tracing instrumentations, Faro web-tracing currently requires that we call getDefaultOtelInstrumentations() with the same arguments as https://github.com/grafana/faro-web-sdk/blob/main/packages/web-tracing/src/instrumentation.ts#L102. However, this is not possible as getIgnoreUrls() is a private function, so calling faro.getIgnoreUrls() is not possible from outside of the class.

As an example, today I was trying to add a couple of opentelemetry/instrumentation into our app (which does not already use opentelemetry on the frontend). To do this I expected I could just add them to the instrumentation list, like this:

initializeFaro({
  instrumentations: [
    ...getWebInstrumentations(),
    new TracingInstrumentation({
      instrumentations: [
        ...getDefaultOtelInstrumentations(),
        new DocumentLoadInstrumentation(),
        new UserInteractionInstrumentation(),
      ],
    })
  ],
})

I found as a result of this that Faro does not get the correct values for ignoreUrls or propagateTraceHeaderCorsUrls.

Proposed solution

I think one way around this -- while maintaining backwards compatibility -- would be to allow an array of additional instrumentations to be given to Faro at initialization time, so that we don't have to re-implement getIgnoreUrls, make getIgnoreUrls public, or re-implement the entire initialize() function.

Then, options.instrumentations would clearly be an override, while options.additionalInstrumentations would be to add more tracing instrumentations that are not included by default.

Example

initializeFaro({
  instrumentations: [
    ...getWebInstrumentations(),
    new TracingInstrumentation({
      instrumentationOptions: {
         propagateTraceHeaderCorsUrls: [ ... ],
      }
      additionalInstrumentations: [
        new DocumentLoadInstrumentation(),
        new UserInteractionInstrumentation(),
      ],
    })
  ],
})

Context

This code block here is responsible for the default setup of OTEL instrumentations and configuration: https://github.com/grafana/faro-web-sdk/blob/main/packages/web-tracing/src/instrumentation.ts#L99-L108