Add settle spans from OTEP 220

Author: pyohannes

docs/messaging/messaging-spans.md

@@ -206,6 +206,7 @@ The following operations related to messages are defined for these semantic conv
 | `create`       | A message is created. "Create" spans always refer to a single message and are used to provide a unique creation context for messages in batch publishing scenarios. |
 | `receive`      | One or more messages are requested by a consumer. This operation refers to pull-based scenarios, where consumers explicitly call methods of messaging SDKs to receive messages. |
 | `deliver`      | One or more messages are passed to a consumer. This operation refers to push-based scenarios, where consumer register callbacks which get called by messaging SDKs. |
+| `settle`       | One or more messages are settled. |
 
 ### Span kind
 
@@ -269,6 +270,12 @@ batch of messages, or for no message at all (if it is signalled that no
 messages were received). For each message it accounts for, the "Deliver" or
 "Receive" span SHOULD link to the message's creation context.
 
+"Settle" spans SHOULD be created for every manually or automatically triggered
+settlement operation. A single "Settle" span can account for a single message
+or for multiple messages (in case messages are passed for settling as batches).
+For each message it accounts for, the "Settle" span MAY link to the creation
+context of the message.
+
 ## Messaging attributes
 
 Messaging attributes are organized into the following namespaces: