Add consumer_id to identify the consumer

Resolves open-telemetry#1796

semantic_conventions/trace/messaging.yaml

@@ -57,6 +57,14 @@ groups:
             The [conversation ID](#conversations) identifying the conversation to which the message belongs,
             represented as a string. Sometimes called "Correlation ID".
           examples: 'MyConversationId'
+        - id: consumer_id
+          type: string
+          brief: >
+            The identifier for the consumer receiving a message. For Kafka, set it to
+            {`messaging.kafka.consumer_group` - `messaging.kafka.client_id`}, if both are present, or only
+            `messaging.kafka.consumer_group`. For brokers, such as RabbitMQ and Artemis, set it to the `client_id`
+            of the client consuming the message.
+          examples: 'mygroup - client-6'
         - id: message_payload_size_bytes
           type: int
           brief: >

specification/trace/semantic_conventions/messaging.md

@@ -136,6 +136,7 @@ The following operations related to messages are defined for these semantic conv
 | `messaging.url` | string | Connection string. | `tibjmsnaming://localhost:7222`; `https://queue.amazonaws.com/80398EXAMPLE/MyQueue` | No |
 | `messaging.message_id` | string | A value used by the messaging system as an identifier for the message, represented as a string. | `452a7c7c7c7048c2f887f61572b18fc2` | No |
 | `messaging.conversation_id` | string | The [conversation ID](#conversations) identifying the conversation to which the message belongs, represented as a string. Sometimes called "Correlation ID". | `MyConversationId` | No |
+| `messaging.consumer_id` | string | The identifier for the consumer receiving a message. For Kafka, set it to {`messaging.kafka.consumer_group` - `messaging.kafka.client_id`}, if both are present, or only `messaging.kafka.consumer_group`. For brokers, such as RabbitMQ and Artemis, set it to the `client_id` of the client consuming the message. | `mygroup - client-6` | No |
 | `messaging.message_payload_size_bytes` | int | The (uncompressed) size of the message payload in bytes. Also use this attribute if it is unknown whether the compressed or uncompressed payload size is reported. | `2738` | No |
 | `messaging.message_payload_compressed_size_bytes` | int | The compressed size of the message payload in bytes. | `2048` | No |
 | [`net.peer.ip`](span-general.md) | string | Remote address of the peer (dotted decimal for IPv4 or [RFC5952](https://tools.ietf.org/html/rfc5952) for IPv6) | `127.0.0.1` | If available. |