Consider additional telemetry devices for deeper Solr observability

## Summary
Several useful Solr and OS metrics are not yet collected as telemetry devices.
This issue tracks candidates for future implementation.

## Candidate devices

| Device | Description |
|--------|-------------|
| `SolrReplicationStats` | Replication lag tracking between leader and replicas — useful for NRT/TLOG replica benchmarks |
| `StartupTime` | Node startup duration (time from launch to accepting queries) — important for cold-start benchmarks |

## Next steps
Each device should be discussed and tracked in its own issue once prioritised.