Add BetterDB Monitor to Databases exporters list

[KIvanow]

What changed

docs/instrumenting/exporters.md: add BetterDB Monitor to the Databases list, between AWS RDS exporter and ClickHouse exporter.

Why

BetterDB Monitor is a monitoring dashboard for Valkey, Redis and Dragonfly, available self-hosted (Docker/Helm/npm) or as a hosted cloud service. It exports 110+ Prometheus metrics scraped from INFO, CLUSTER SLOT-STATS, slowlog/COMMANDLOG history and its own anomaly detectors (metrics reference). There is currently no Valkey-related entry on this page. The core is MIT-licensed (open-core; some features are commercial).

Sample output, scraped from a live instance monitoring Valkey 8.1.8:

# HELP betterdb_uptime_in_seconds Server uptime in seconds
# TYPE betterdb_uptime_in_seconds gauge
betterdb_uptime_in_seconds{connection="localhost:6380"} 149
# HELP betterdb_instance_info Instance information (always 1)
# TYPE betterdb_instance_info gauge
betterdb_instance_info{connection="localhost:6380",version="8.1.8",role="master",os="Linux 6.6.144-3-MANJARO x86_64"} 1
# HELP betterdb_memory_used_bytes Total allocated memory in bytes
# TYPE betterdb_memory_used_bytes gauge
betterdb_memory_used_bytes{connection="localhost:6380"} 1353824
# HELP betterdb_keyspace_hits_total Total keyspace hits
# TYPE betterdb_keyspace_hits_total gauge
betterdb_keyspace_hits_total{connection="localhost:6380"} 0

All metrics carry a connection label; one instance can monitor several databases.

cc @RichiH