Add ability for SpanProcessor to mutate spans on end

CHANGELOG.md

@@ -11,6 +11,9 @@ release.
 
 ### Traces
 
+- Add `OnEnding` callback to SDK `SpanProcessor` interface
+  ([#4024](https://github.com/open-telemetry/opentelemetry-specification/pull/4024))
+
 ### Metrics
 
 ### Logs

specification/trace/sdk.md

@@ -38,6 +38,7 @@ linkTitle: SDK
 - [Span processor](#span-processor)
   * [Interface definition](#interface-definition)
     + [OnStart](#onstart)
+    + [OnEnding](#onending)
     + [OnEnd(Span)](#onendspan)
     + [Shutdown()](#shutdown)
     + [ForceFlush()](#forceflush)
@@ -565,7 +566,8 @@ in the SDK:
 
 `OnStart` is called when a span is started. This method is called synchronously
 on the thread that started the span, therefore it should not block or throw
-exceptions.
+exceptions. If multiple `SpanProcessors` are registered, their `OnStart` callbacks
+are invoked in the order they have been registered.
 
 **Parameters:**
 
@@ -580,6 +582,25 @@ exceptions.
 
 **Returns:** `Void`
 
+#### OnEnding
+
+**Status**: [Experimental](../document-status.md)
+
+`OnEnding` is called just before a span is ended. The span is still mutable, but the end timestamp is already set.
+This method MUST be called synchronously within the [`Span.End()` API](api.md#end),
+therefore it should not block or throw an exception.
+If multiple `SpanProcessors` are registered, their `OnEnding` callbacks
+are invoked in the order they have been registered.
+The SDK MUST guarantee that the span cannot be modified anymore by any other thread
+before invoking `OnEnding` of the first `SpanProcessor`. From that point on, modifications
+are only allowed synchronously from within the invoked `OnEnding` callbacks.
+
+**Parameters:**
+
+* `span` - a [read/write span object](#additional-span-interfaces) for the span which is about to be ended.
+
+**Returns:** `Void`
+
 #### OnEnd(Span)
 
 `OnEnd` is called after a span is ended (i.e., the end timestamp is already set).