fix: query announced endpoints match relabel-config

[verejoel]

The --selector.relabel-config flag was only filtering stores during query execution, but not when announcing available label sets via the Info API or /stores endpoint. This caused a mismatch where:

• Queries correctly returned filtered results
• UI and inter-querier communication showed unfiltered label sets

This happens because ProxyStore's LabelSet() method ignored filtered results from TSDBSelector.MatchLabelSets() and always included all label sets from matching stores.

In this PR, label set announcements now match the actual queryable data, fixing confusion in multi-tier querier deployments where root queriers would see all tenant labels despite leaf queriers being configured to filter specific tenants.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• Apply TSDBSelector filtering consistently in ProxyStore.LabelSet() and TimeRange()
• Filter /stores REST endpoint using same TSDBSelector logic
• Pass TSDBSelector to QueryAPI for UI consistency

Verification

I have this working in our DEV env. We have a leaf querier that's setup with a hashmod relabel config across our ingester fleet.

For simplicity, I show this with a keep relabel config as well (both work)

        - --endpoint=dnssrvnoa+_grpc._tcp.thanos-ingester.thanos.svc.cluster.local
        - |
          - action: keep
            source_labels: [tenant_id]
            regex: sdp

We also have a root querier plugged into all leaf querier shards.

        - --endpoint=thanos-ingester-querier-0.thanos.svc.cluster.local:10901
        - --endpoint=thanos-ingester-querier-1.thanos.svc.cluster.local:10901
        - --endpoint=thanos-ingester-querier-2.thanos.svc.cluster.local:10901

Without the patch, in both the leaf and root queriers, the /stores endpoint shows all external labels on every endpoint.

With the patch, the result is more expected:

[fpetkovski]

Awesome work 👍