Span is already immutable during OnEnding span processor callbacks, violating the spec

Describe your environment

OS: any
Python version: any
SDK version: 1.44.0 (present since the hook was added in #4775)
API version: 1.44.0

What happened?

The trace SDK spec says the span MUST still be mutable while OnEnding span processor callbacks run — the whole point of the hook is to allow last-moment modifications before export, with only the end timestamp already set:

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#onending

In the Python SDK, Span.end() freezes the span before invoking _on_ending: it sets _end_time and marks the attributes immutable first, then calls the callbacks. All mutators (set_attribute, set_attributes, add_event, add_link, update_name, set_status) treat _end_time is not None as "span has ended" and silently no-op (they only log a warning), and is_recording() returns False. As a result, every mutation attempted from an _on_ending callback is dropped and never reaches the exported span, making the hook observe-only.

Steps to Reproduce

from opentelemetry.sdk.trace import TracerProvider, SpanProcessor
from opentelemetry.sdk.trace.export import SimpleSpanProcessor
from opentelemetry.sdk.trace.export.in_memory_span_exporter import InMemorySpanExporter


class MyProcessor(SpanProcessor):
    def _on_ending(self, span):
        span.set_attribute("on_ending.attr", "value")


exporter = InMemorySpanExporter()
provider = TracerProvider()
provider.add_span_processor(MyProcessor())
provider.add_span_processor(SimpleSpanProcessor(exporter))

provider.get_tracer(__name__).start_span("foo").end()

(span,) = exporter.get_finished_spans()
print(span.attributes)  # {} — the attribute set in _on_ending is gone

Expected Result

The exported span contains on_ending.attr, and is_recording() returns True while the callback runs.

Actual Result

The mutation is silently dropped (Setting attribute on ended span. warning) and the exported span has no attributes.

Would you like to implement a fix?

Yes

Start at Span.end() and the _on_ending callback path in the Python SDK, then compare their behavior with the linked OpenTelemetry specification. Reproduce the issue using the provided MyProcessor example and verify that mutations are retained, is_recording() is true during the callback, and the exported span contains on_ending.attr.