List invalid routes via support listener

[AlexanderYastrebov]

Is your feature request related to a problem? Please describe.

In multitenant ingress setting users may define invalid routes and operators may have to figure out where invalid routes are coming from. Currently it is possible via monitoring logs for a specific error message.

Describe the solution you would like

Skipper serves valid routes via support listener API at /routes.
It also have a list of invalid routes so we may add a support API say at /invalid-routes to show them as well.
Along with a new metric for number of invalid routes this could be used for alerting and debugging invalid user configurations.

Describe alternatives you've considered (optional)

Monitor logs for a specific error message.

Additional context (optional)

The problem is partially addressed by a webhook which prevents users from creating routes with e.g. invalid eskip syntax but it does not (currently) prevent users from creating non-existing or disabled filters.

Would you like to work on it?

This is a good first issue for someone :)

[szuecs]

Idk API related: maybe /routes/invalid or a query: /routes?invalid=true or /routes?state=invalid.
Maybe path is better but I want to discuss some options and think which one is best and why.

Query is good for filtering but if you know skipper well you also know that invalid routes are dropped and not part of the active routing table.
From API point of view it might make sense to use the query instead of path.

[AlexanderYastrebov]

We may also expose corresponding error message per invalid route.