docs(messaging): add gcp async batch publish example

CHANGELOG.md

@@ -36,6 +36,8 @@ release.
   ([#21](https://github.com/open-telemetry/semantic-conventions/pull/21))
 - Add `messaging.gcp_pubsub.message.ordering_key` attribute.
   ([#528](https://github.com/open-telemetry/semantic-conventions/pull/528))
+- Add an example for gcp_pubsub asynchronous batch publish
+  ([#548](https://github.com/open-telemetry/semantic-conventions/pull/545)).
 - Define how to set `process.runtime.name`, `process.runtime.version`,
   `process.runtime.description` for .NET runtime.
   ([#561](https://github.com/open-telemetry/semantic-conventions/pull/561))

docs/messaging/gcp-pubsub.md

@@ -19,4 +19,42 @@ For Google Cloud Pub/Sub, the following additional attributes are defined:
 | [`messaging.gcp_pubsub.message.ordering_key`](../attributes-registry/messaging.md) | string | The ordering key for a given message. If the attribute is not present, the message does not have an ordering key. | `ordering_key` | Conditionally Required: If the message type has an ordering key set. |
 <!-- endsemconv -->
 
+## Examples
+
+### Asynchronous Batch Publish Example
+
+Given is a process P that asynchronously publishes 2 messages in a batch to a topic T on Pub/Sub.
+
+```mermaid
+flowchart LR;
+  subgraph PRODUCER
+  direction LR
+  CA[Span Create A]
+  CB[Span Create B]
+  P[Span Publish A B] 
+  end
+  CA-. link .-P;
+  CB-. link .-P;
+
+  classDef producer fill:green
+  class P,CA,CB producer
+  classDef normal fill:green
+  class PA,PB,D1 normal
+  linkStyle 0,1 color:green,stroke:green
+```
+
+| Field or Attribute | Span Create A | Span Create B | Span Publish A B |
+|-|-|-|-|
+| Span name | `T create` | `T create` | `publish` |
+| Parent |  |  |  |
+| Links |  |  | Span Create A, Span Create B |
+| SpanKind | `PRODUCER` | `PRODUCER` | `CLIENT |
+| Status | `Ok` | `Ok` | `Ok` |
+| `messaging.batch.message_count` |  |  | 2 |
+| `messaging.destination.name` | `"T"` | `"T"` |  |
+| `messaging.operation` | `"create"` | `"create"` | `"publish"` |
+| `messaging.message.id` | `"a1"` | `"a2"` | |
+| `messaging.message.envelope.size` | `1` | `1` | |
+| `messaging.system` | `"gcp_pubsub"` | `"gcp_pubsub"` | |
+
 [DocumentStatus]: https://github.com/open-telemetry/opentelemetry-specification/tree/v1.26.0/specification/document-status.md