Allow overriding or customizing span.RecordError() behavior globally

[Bhogayata-Keval]

Summary
I’d like to propose a mechanism to allow users to override or hook into span.RecordError() globally — for example, to enrich error-related span events with custom attributes consistently across all instrumented libraries.

Motivation
Currently, there is no clean way to globally customize the behavior of span.RecordError().

In my use case, I want to inject an additional attribute (e.g., custom.source = "internal") into every exception span event, regardless of which instrumentation library triggered it (Gin, Mux, Fiber, etc.).

While it’s technically possible to:

Manually call span.AddEvent("exception", ...) with custom attributes, or

Wrap span.RecordError() in user code,

…these solutions are not scalable across large codebases or libraries using auto-instrumentation.

What I tried
SpanProcessor: Can observe exception events in OnEnd(), but cannot mutate span data (it's read-only).

SpanExporter: Allows exporting enriched data, but this requires duplicating span data structures or intercepting and transforming OTLP payloads — which feels heavy-handed for a relatively simple customization.

Proposal
Introduce a hook or registry mechanism (e.g., otel.SetErrorRecorder(...)) where users can globally define how RecordError() enriches the span. This would preserve the default behavior while allowing teams to attach custom tags, redactions, or formatting logic.

[dmathieu]

This would need a specification change.

[dmathieu]

While this SDK doesn't implement it yet, the (unstable) OnFinish hook for processors would be a good place to mutate spans if they include an error.
I think a custom processor using OnFinish would be better than a custom hook on RecordError (why not also on Start, AddAttributes, ...?)

[MrAlias]

I'm not in favor of global configuration to control this

+1 to custom processors or writing your own SDK to perform your desired behavior.