Expose tenancy-aware /api/v1/labels and /api/v1/label/<name>/values endpoints for Thanos query

prom-label-proxy v0.3.0 has a new flag called --enable-labels-api [0] which allows to expose /api/v1/labels and /api/v1/label/<name>/values endpoints [1]. It would be useful to enable this flag for the prom-label-proxy instance which enforces namespace isolation in front of Thanos query. One use case is that people may want to use Grafana with the tenancy port of Thanos query and Grafana queries these endpoints for dashboard templating (see [2] for an example). The dev console might also leverage the endpoints instead of querying for all metrics names in a given namespace [3].

DoD

• /api/v1/labels and /api/v1/label/<name>/values endpoints are accessible from
  thanos-querier.openshift-monitoring.svc:9092.
• prom-label-proxy injects the namespace matcher in the API query and the returned data is scoped to the namespace value.

[0] https://github.com/prometheus-community/prom-label-proxy/#metadata-endpoints
[1] https://prometheus.io/docs/prometheus/latest/querying/api/#getting-label-names
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1969409
[3] https://github.com/openshift/console/blob/85086217feb35e64c36d68a8d315fe59275e510b/frontend/packages/dev-console/src/components/monitoring/metrics/MetricsQueryInput.tsx#L91-L97