apmotel: add nil and recording checks to span.RecordError

[jacksehr]

Addresses feature request #1565

From it:

The apmotel bridge has proved extremely helpful in migrating to OpenTelemetry spans/tracers, but there is one noticeable difference inside the Span implementation: RecordError.

The OpenTelemetry SDK implementation of span.RecordError() includes a nil check for the error that is passed in.

In contrast, the apmotel implementation does no such check. While this is understandable, and arguably more idiomatic, it introduces an issue where you cannot seamlessly drop in the bridge and use span.RecordError how you might expect to with the OpenTelemetry SDK.

This PR implements the mentioned nil/recording checking.

[simitt]

Thank you for the contribution @jacksehr! Changes look reasonable to me for otel alignment.

[dmathieu]

@elasticmachine run elasticsearch-ci/docs

[shravanjeevan]

@dmathieu Would it be possible to get a minor version release with this change? We are waiting on it to progress some things. Thanks 🙏

[dmathieu]

I have released 2.4.8.