Adapt KeycloakMetricsHandler to consider custom relative paths via `kc.http.relative-path`
or --http-relative-path=/auth. Previously only the fixed path /metrics was allowed which did not work
with custom context-paths.
- Adapted KeycloakProcessor to Pedros proposal from 0b28d71dd0
- Removed unnecessary KeycloakMetricsHandler
- Added MetricsDistTest
Fixes#8877