feat(spans): Attachment V2 envelope item (#15466)

Author: jjbayer

develop-docs/sdk/telemetry/spans/span-protocol.mdx

@@ -220,3 +220,52 @@ Example:
 trace_id: "6cf173d587eb48568a9b2e12dcfbea52"
 span_id: "438f40bd3b4a41ee"
 ```
+
+## Span Attachments
+
+The SDK must implement a new "attachment v2" envelope item, which is used to emit span attachments to Sentry. A span attachment should be emitted in the same envelope as its owner span, so that it can be dropped when the span itself is filtered or rate limited.
+
+### Attachment v2 Envelope Item Header
+
+The envelope item header must contain the following properties:
+
+```json
+{
+  "type": "attachment",
+  "content_type": "application/vnd.sentry.attachment.v2",
+  "length": 212341234,
+  "meta_length": 123,
+  "span_id": "abcd1234" // or `null`
+}
+```
+
+- `meta_length` is the size of the metadata portion of the payload in bytes (see item description below).
+- `span_id` is the ID of the span that owns the attachment. `span_id: null` is treated as “owned by spans”, but not owned by a specific span. That is, the attachment can be dropped if the span quota is exceeded, but it will not be dropped with a specific span because of e.g. inbound filters.
+
+## Attachment v2 Envelope Item Payload
+
+The attachment item payload consists of JSON object containing metadata followed by the attachment body. For example, for a plain text attachment with the body "Hello World!", the item payload would look like this:
+
+```json
+{ // Attachment Metadata
+  "attachment_id": "019a72d07ffe77208c013ac888b38d9e",
+  "timestamp": 1760520026.781239,
+  "filename": "myfile.txt",
+  "content_type": "text/plain",
+	"attributes": {
+	  // Arbitrary key value pairs that will end up in EAP.
+		"foo": {"type": "string", "value": "bar"}
+	}
+}helloworld
+```
+
+| Property | Type | Required | Description |
+|----------|------|----------|-------------|
+| `attachment_id` | string | Yes | 32-character hexadecimal string (a valid [Version 7](https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_7_(timestamp_and_random)) or Version 4 UUID without dashes) |
+| `timestamp` | float | Yes | a UNIX timestamp corresponding to the attachment's occurrence. It may be the same as the start or end timestamp of the owner span. |
+| `filename` | string | No | The file name of the attachment. |
+| `content_type` | string | Yes | the content type of the attachment body (not to be confused with the content type of the envelope item, which is always `application/vnd.sentry.attachment.v2`). |
+| `attributes` | object | No | arbitrary attributes that will be queryable on the attachment trace item, similar to spans, logs, and trace metrics. |
+
+
+