Document backend SDKs metrics requirements (#15537)

Author: cleptric

develop-docs/sdk/telemetry/metrics.mdx

@@ -425,6 +425,18 @@ By default, Relay should parse the user agent attached to an incoming metric env
 }
 ```
 
+#### Backend SDKs
+
+For backend SDKs (Node.js, Python, PHP, etc.), the SDKs should attach the following:
+
+1. `server.address`: The address of the server that sent the log. Equivalent to [`server_name`](sdk/data-model/event-payloads/#optional-attributes) we attach to errors and transactions.
+
+```json
+{
+  "server.address": "foo.example.com"
+}
+```
+
 #### Future Default Attributes
 
 The SDKs should aim to minimize the number of default attributes attached to a metric. Metrics cardinality can explode quickly with too many attributes, and we want to keep metrics lightweight and cost-effective.