Metric unit is required, see: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/cmd/mdatagen/metadata-schema.yaml#L72-L73.

This PR adds the unit to metrics where this is missing:
- redis.role
- redis.cmd.calls
- redis.clients.connected
- redis.clients.blocked
- redis.keys.expired
- redis.keys.evicted
- redis.connections.received
- redis.connections.rejected
- redis.memory.fragmentation_ratio
- redis.rdb.changes_since_last_save
- redis.commands.processed
- redis.keyspace.hits
- redis.keyspace.misses
- redis.slaves.connected
- redis.db.keys
- redis.db.expires