[Observability] Observability app pages seem to all show a blank (seemingly broken) white screen for a non-existent route

[jasonrhodes]

Observability apps do not seem to attempt to handle a 404 state for a non-existent route.

See some examples

Whereas in /app, these are handled with a 404 screen:

@smith also mentions:

it looks like behavior across apps is not consistent. /app/security/blah redirect to the overview, /app/home/blah shows that url but it’s just the home page. /app/enterprise_search/what shows application not found. /app/fleet/what redirects to /app/fleet/agents, and so on

So it's not something we handle consistently across Kibana, but we should try to be consistent within Observability and avoid the blank white screen scenario, at the very least.

AC:

We show a consistent non-blank screen for all non-existent routes across Observability apps. This should be handled at the router level for each plugin.

• APM [APM] Add 404 page #136214
• Infra Logs and Metrics UI 404 pages #136335
• Uptime
• Observability
• Stack monitoring

Thank you to @cchaos for pointing this out.

[afgomez]

I have created a shared <NotFound /> component in #145598 based on the EUI Guidelines for this. It can be used anywhere where an <EuiPrompt /> can be used.