Remove progress data for completed phases from metrics

Problem

The /metrics endpoint continues to report progress data for bootstrap phases that have already completed. For example, after sync is complete:

{
  "bootstrapPhase": "synced",
  "headerSyncProgress": {
    "currentSlot": 103092890,
    "targetSlot": 103093009
  },
  "extractionProgress": {
    "current": 2900993,
    "eta": 0,
    "percent": 100,
    "rate": 550.58,
    "total": 2900993
  }
}

The headerSyncProgress and extractionProgress fields are stale and no longer relevant once the service is in the synced phase.

Proposal

Remove or null out progress data for phases that have completed:

• When bootstrapPhase is synced, omit headerSyncProgress and extractionProgress
• When bootstrapPhase is syncing_headers, omit extractionProgress
• Only show progress data relevant to the current phase

This makes the metrics cleaner and avoids confusion about which data is current.