Update 'unit' property description in metrics documentation (#16009)

Clarified the 'unit' property description for metrics.

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Author: adinauer

develop-docs/sdk/telemetry/metrics.mdx

@@ -206,7 +206,8 @@ The parameters for these methods should be the following, or an equivalent form
 - `name` **String, required**: The name of the metric
 - `value` **Number, required**: The value of the metric
 - `options` **Object, optional**: An object containing the following properties:
-  - `unit` **String, optional**: The unit of measurement (only used for `distribution` and `gauge`)
+  - `unit` **String, optional**: The unit of measurement (only used for `distribution` and `gauge`). SDKs must not restrict what can be sent in (e.g. by using an enum) but should offer constants or similar that help customers send in <Link
+to="/sdk/telemetry/attributes/#units">units we support</Link>.
   - `attributes` **Object, optional**: A dictionary of attributes (key-value pairs with type information)
   - `scope` **Scope, optional**: The scope to capture the metric with.