[Stack Monitoring][Angular removal] Blank page between loading page and first page

[estermv]

When users land on the stack monitoring page they see a blank page between "loading states".

This happens because the flow is:

1. User go to stack monitoring and is redirected to the /loading page
2. User is redirected to /overview
3. The component does a request to the clusters endpoint
4. While it waits for that request to finish, users see a blank page.

We could render a loading spinner while the request doesn't finish.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)