mgr/dashboard: fix memory leak in prometheus service

mgr/dashboard: fix memory leak in prometheus service

Prometheus API calls in the Cluster Utilization call is subscribed in
the for loop multiple times but this is not properly unsubscribed. As we
stay in the dashboard page for longer time, it produces a significant
memory leak which eventually lags the UI. Attempting to fix it by
properly handling the subscription

Fixes: https://tracker.ceph.com/issues/72511
Signed-off-by: Nizamudeen A <nia@redhat.com>
(cherry picked from commit 26765720b972453bc69f23cd5644366bba998391)