Allow NodeSDK to specify sampler without spanProcessor/exporter

[dvoytenko]

What happened?

Steps to Reproduce

const sdk = new NodeSDK({
   sampler: new MySampler(),
});

Expected Result

The specified sampler is used.

Actual Result

Default sampler is initialized.

Additional Details

This is due to this code: the specified sampler is only accepted when the config also contains a spanProcessor or a traceExporter. However, these concepts are not necessarily require each other. It's should be ok to configure one without the other.

OpenTelemetry Setup Code

import { NodeSDK, type NodeSDKConfiguration } from '@opentelemetry/sdk-node';
const sdk = new NodeSDK({
  sampler: new MySampler(), // Never called.
});
sdk.start();

### package.json

```JSON
{
  "dependencies": {
    "@opentelemetry/sdk-node": "^0.44.0"
  }
}

Relevant log output

No response

[dyladan]

This means a sampler cannot be configured when using environment variables to set up the export pipeline.

[dvoytenko]

@dyladan Is this a statement for or against supporting sampler without processor/exporter configuration?

[dyladan]

It's just a statement of the current state sorry. I should have added more context. I think this is something we should fix, but I think it is a feature request and not a bug.

[dyladan]

This actually is a bug the more I think about it. It means the sampler is not being applied correctly.

[WesleyYue]

I think this is a duplicate of #4004

[pichlermarc]

This seems to have been fixed by #4394 - merging the PR did not close this issue.