Add support for OnEnding() method to SpanProcessor

[simonpasquier]

Problem Statement

The OpenTelemetry trace SDK spec recently added the ability for SpanProcessor to mutate spans on end (open-telemetry/opentelemetry-specification#4024 and https://opentelemetry.io/docs/specs/otel/trace/sdk/#onending).

AFAICT the feature isn't yet supported by the Go SDK (https://pkg.go.dev/go.opentelemetry.io/otel/sdk@v1.35.0/trace#SpanProcessor).

Proposed Solution

NA

Alternatives

NA

Prior Art

NA

Additional Context

This feature has been mentioned as a possible solution for #5703. It's also been mentioned in Azure/azure-sdk-for-go#24347.

[dmathieu]

See #5756 for context on the challenges this gives, and how it's not a trivial change.