Cardinality issue with non-existent client

[rekup]

Description

This extension exposes several metrics about attempted logins to non-existent clients:

• keycloak_failed_login_attempts_created
• keycloak_failed_login_attempts_total
• keycloak_login_attempts_created
• keycloak_login_attempts_total

The problem is that if some script kiddie scans keycloak they might use any data as client id which will cause a huge number metrics:

Expected Behavior

I would expect that this extension only creates metrics for existing clients and maybe exposes one metric which records all the events for non-existent clients.

Environment

• keycloak-metrics-spi: 5.0.0
• Keycloak: 23.0.7

Steps to reproduce

1. Start keycloak with the keycloak-metrics-spi extension
2. Make a auth request to a non-existent client: https://sso.example.org/auth/realms/my-realm/protocol/openid-connect/auth?client_id=foo&redirect_uri=https%3A%2F%2Ffoo.example.org&response_type=code&state=1234
3. Check the generated metrics using promql: sum by(__name__, client_id) ({client_id="foo1"})

[tolleiv]

@pb82 @aerogear are you open for contributions for this one?