Alternative to deprecated addSpanProcessor

[satazor]

Is your feature request related to a problem? Please describe

I'm using the OpenTelemetry Node SDK to initialize OpenTelemetry because it does most of the work of setting it up. Besides the default span processor, I want to add some more span processors. I was using addSpanProcessor but it was deprecated here: open-telemetry/opentelemetry-js#5134

At first, spanProcessors constructor option of NodeSDK seems designed for this use-case, but it has a shortcoming: you must specify the extra ones plus the default BatchSpanProcessor. However, the BatchSpanProcessor requires an instance of exporter, that it's created automatically for us via environment variables. Currently, it's not feasible to create an exporter based on environment variables without duplicating code, since the code that does it is hidden via private functions, e.g.: _buildExporterFromEnv and _getSpanExporter within BasicTracerProvider class.

Describe the solution you'd like to see

I would like an easy way to add span processors without having to duplicate the initialization of BatchSpanProcessor and the initialization of the exporter based on environment variables.

Approach 1:
Allow spanProcessors to be a function in order to append / prepend processors:

new NodeSDK({
  spanProcessors: (spanProcessors) => [mySpanProcessor, ...spanProcessors],
});

The same logic could be applied to logRecordProcessors .

Approach 2:
Export the necessary functionality to create stuff from env, such as exposing _buildExporterFromEnv, _buildPropagatorFromEnv, etc.

Describe alternatives you've considered

• Keep using the deprecated addSpanProcessor for now.
• Duplicate the code necessary to build an exporter from env to set up BatchSpanProcessor