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 (https://github.com/open-telemetry/opentelemetry-specification/pull/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/[email protected]/trace#SpanProcessor).

Proposed Solution

NA

Alternatives

NA

Prior Art

NA

Additional Context

This feature has been mentioned as a possible solution for https://github.com/open-telemetry/opentelemetry-go/issues/5703. It's also been mentioned in https://github.com/Azure/azure-sdk-for-go/issues/24347.