Consider additional telemetry devices for deeper Solr observability

[janhoy]

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.