Metrics: ignore estimated bloat for relations < 1 MiB + adjust texts "bloat" --> "estimated bloat"

This MR refines the estimated bloat metrics to reduce noise from very small relations and clarifies wording in metric docs and Grafana UI.

Changes

• Bloat metrics noise reduction

  • Add a > 1 MiB size filter to:
    • pg_btree_bloat (estimated index bloat)
    • pg_table_bloat (estimated table bloat)
  • Inline SQL comments explain the cutoff.

• Terminology / UX

  • Update metric descriptions to say “estimated bloat” (not “bloat”) since results are heuristic and can be off.
  • Update Grafana dashboard wording:
    • Dashboard_10_Index health: “Bloat %/size” → “Estimated bloat %/size”
    • Dashboard_7_Autovacuum_and_bloat: title → “Autovacuum and estimated bloat”

Scope

• No metric names or storage schema changes; this is filtering + labeling only.

Why

• Small relations often produce misleading “bloat” signals; filtering improves signal-to-noise.
• “Estimated bloat” wording prevents overconfidence in heuristic results.

#63