redis.go

Remove `mem_fragmentation_ratio`:
* Comes from `redis_memory_used_rss_bytes / redis_memory_used_bytes`.
* Metric only provides 2 digits of accuracy compared to calculation.

Add allocator metrics.

Cleanup Prometheus naming of commands metrics names to avoid confusing
references to being a "Summary" metric, it's not.

Remove `total_system_memory`, this duplicates node_exporter metrics.

Signed-off-by: Ben Kochie <superq@gmail.com>