Improve and expand on the definition of message.id

specification/data-events.md

@@ -25,18 +25,41 @@ Message events MUST be associated with a tracing span.
 | Attribute name | Notes and examples                     | Required? |
 | :------------- | :------------------------------------- | --------- |
 | `message.type` | Either `"SENT"` or `"RECEIVED"`.       | Yes |
-| `message.id`   | Incremented integer value within the parent span starting with `1`. | Yes |
+| `message.id`   | Unique identifier within a span and `message.type` for the individual message. Further specifications for common protocols are discussed below. | No |
 | `message.compressed_size` | Compressed size in bytes. | No |
 | `message.uncompressed_size` | Uncompressed size in bytes. | No |
 | `message.content` | The body or main contents of the message. If binary, then message should be Base64 encoded. | No |
 
-The `message.id` MUST be calculated as two different counters starting from `1`,
-the first for sent messages and the second for received messages. This way we
-guarantee that the values will be consistent between different implementations.
-In protocols where the message id is included as a header in the message, the
-received message id should be that sent be the server instead of its own
-incremented value.
+To conserve bandwith and/or storage, exporters MAY drop the `message.content`
+attribute if present. Logging-only exporters bundled with default OpenTelemetry
+SDKs SHOULD provide affordances for logging this information as it is highly
+useful during the early stages of developing a new application.
 
-Most exporters will likely drop the `message.content` attribute if present.
-However, logging-only exporters will likely want to log it as this information
-is highly useful during the early stages of developing a new application.
+### Representative message.id attribute values
+
+#### gRPC
+
+For [gRPC], `message.id` MUST be calculated as two different counters starting
+from `1`, the first for sent messages and the second for received messages.
+This way we guarantee that the values will be consistent between different
+implementations. Server streaming, client streaming and bidirectional streaming
+RPCs provided gRPC all guarantee message ordering so there should be no
+discrepencies between the sender and receiver spans.
+
+#### Server-Sent Events
+
+For [Server-Sent Events] included in the HTML5 specification, `message.id` MUST
+match the `id` field of an event if it does not contain U+0000 NULL. Otherwise,
+the attribute should not be populated. This is equivalent to what gets populated
+to the last event ID buffer of the `EventSource` object specified in the SSE
+specification.
+
+#### Java Message Service (JMS)
+
+For JMS, `message.id` SHOULD be set to the value of the `JMSMessageID`field
+exposed by [JMS Message]. This value uniquely identifies each message sent by
+a provider.
+
+[gRPC]: https://www.grpc.io/docs/guides/concepts/
+[Server-Sent Events]: https://html.spec.whatwg.org/multipage/server-sent-events.html
+[JMS Message]: https://docs.oracle.com/javaee/7/api/javax/jms/Message.html