Fix epoch uint64 to string conversion in metrics labels

[Copilot]

Addresses feedback from #64 on incorrect epoch conversion that interpreted uint64 values as Unicode code points instead of numeric strings.

Changes

• Fixed RecordConnectionCount in observability/metrics.go to use strconv.FormatUint(epoch, 10) instead of string(rune(epoch))
• Added strconv import

Example

// Before: epoch 1234 becomes garbage Unicode character
ConnectionsPerEpoch.WithLabelValues(connection, string(rune(epoch)))

// After: epoch 1234 becomes "1234"
ConnectionsPerEpoch.WithLabelValues(connection, strconv.FormatUint(epoch, 10))

This ensures Prometheus metric labels correctly display epoch values as decimal strings.

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