fix(custom-metrics): say "name taken" only for a name clash

The form read every 409 as "a metric with this name already exists".
Now that it keeps one idempotency key across taps, a second tap while
the first create is still running also answers 409, and the form told
the person their name was taken. The create and rename routes send
`customMetric.name_taken` with the clash, and the form matches the code.

src/components/custom-metrics/custom-metric-form.tsx

@@ -132,8 +132,10 @@ export function CustomMetricForm({
       );
       onSuccess?.(saved);
     } catch (err) {
+      // By its code: a 409 can also be the first save still running.
       setError(
-        err instanceof ApiError && err.status === 409
+        err instanceof ApiError &&
+          err.meta?.errorCode === "customMetric.name_taken"
           ? t("customMetrics.form.nameTaken")
           : localizedApiError(err, t, "customMetrics.form.saveError"),
       );