Use routing latency as an APDEX for KAS
This change introduces a basic APDEX score for KAS, see Apdex metrics for KAS (gitlab-org/cluster-integration/gitlab-agent#313 - closed)
The k8s_api_proxy_routing_duration_seconds_bucket bucket for the APDEX is already used in the CI Tunnel Dashboard.
Edited by Timo Furrer