ref(develop/scopes): Clarify unit type for attributes (#15546)

For now we specify units on attributes as follows:
- accept all `MetricUnit`s (see
[Relay](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html))
  - except for `''`, `'none'`
- except for custom units (arbitrary strings). <-- we can revisit this
if we want to widen the type here

Author: Lms24

develop-docs/sdk/telemetry/scopes.mdx

@@ -61,7 +61,9 @@ Users MUST be able to attach attributes to any scope using a dedicated method (e
 Attributes are key-value pairs where each value is either a an attribute value or an object containing:
 
 - `value`: The actual attribute value, which MUST match the specified type
-- `unit` (optional): The unit of measurement (e.g., `"ms"`, `"s"`, `"bytes"`, `"count"`, `"percent"` or any other string value). SDKs MAY NOT add support for units for the moment, but MUST be able to do so at a later date without a breaking change.
+- `unit` (optional): The unit of measurement (e.g., `"millisecond"`, `"second"`, `"byte"`, `"ratio"`).
+  - SDKs MAY delay adding support for units for the moment, but MUST be able to do so at a later date without a breaking change.
+  - SDKS MUST allow setting all units supported by Relay's [`MetricUnit` enum](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html) but MUST NOT allow custom units (i.e. arbitrary strings, defined in [`CustomUnit`](https://getsentry.github.io/relay/relay_metrics/struct.CustomUnit.html)) or "none" units (`''` or `'none'`).
 - `type` (optional): The type of the attribute. SDKs SHOULD NOT expose this to users if they can reliably infer the type from the value. If not, SDKs MAY allow or require users to set the `type` explicitly.
 
 #### Example Usage