What this PR does / why we need it:

Adds rich Prometheus metrics to the Feast feature server covering areas that previously had no observability:

• Request latency histograms with feature_count and feature_view_count labels so operators can correlate latency with request complexity
• Materialization tracking — success/failure counters and duration histograms per feature view
• Feature freshness gauges — staleness (seconds since last materialization) per feature view, updated via background polling
• Online feature retrieval counters — request counts and entity-row-per-request histograms
• Push request counters — tracked by push source and mode (online/offline)
• CPU and memory gauges — moved from inline code to the shared metrics module
  Previously the feature server only had inline CPU/memory monitoring. This PR introduces 10 new metric families covering the full request lifecycle, materialization pipeline, and data freshness.

New metrics

Configuration

Metrics are fully opt-in with zero overhead when disabled. Enable via CLI or YAML:

feature_server:
  metrics:
    enabled: true
    resource: true          # CPU / memory gauges
    request: true           # endpoint latency & request counters
    online_features: true   # online feature retrieval counters
    push: true              # push request counters
    materialization: true   # materialization counters & duration
    freshness: true         # per-feature-view freshness gauges

Per-category toggles let disable specific metric groups (e.g., keep CPU/memory but skip request latency). All categories default to true. The --metrics CLI flag works without any YAML config.