feat: Add feature_service label to Feature Server RED metrics #6639

Follow-up from #5920 (Improve Feature Server Observability).

Problem

sdk/python/feast/metrics.py currently breaks out online-feature metrics by feature_view (see online_features_status_total's feature_view label in track_feature_statuses), but there is no equivalent feature_service label anywhere in the metrics, even though requests to /get-online-features are frequently made via a named Feature Service.

Proposed solution

• Add a feature_service label to the relevant RED metrics (at minimum request_latency/request_count in the track_request_latency context, and consider online_features_status_total).
• Resolve the feature service name in the /get-online-features handler in sdk/python/feast/feature_server.py (available from the parsed request body) and pass it through via RequestMetricsContext, following the existing pattern used for feature_count/feature_view_count.
• Add cardinality safeguards per the original issue: make this label opt-in (config flag, default off) and/or support an allowlist of feature service names, since unbounded feature-service cardinality could blow up metric storage in large deployments.
• Wire the new config flag into MetricsConfig / _MetricsFlags alongside the existing per-category toggles.

cc: @jyejare @ntkathole