Add blob cache usage metrics for Titan

[Connor1996]

Now, we only have blob cache hit metrics for Titan. It would be useful to have a blob cache usage metrics just like block cache usage.

Task breakdown:

• Add an API for rust-rocksdb to get blob_cache_usage, you can refer to how we added block cache usage before set & get block cache capacity via cf_options rust-rocksdb#198
• Collect the blob cache usage metrics in TiKV, see how we do it for block cache usage https://github.com/tikv/tikv/blob/master/components/engine_rocks/src/rocks_metrics.rs#L1071
• Add a blob cache usage panel for Titan in grafana

[Rustin170506]

I am try to working on this.

[Connor1996]

@Rustin-Liu Thanks!
@zhuboshuai Do you have been working on it? If that, please inform @Rustin-Liu