Add RocksDB Block Cache Usage Metrics

[alexanderkiel]

The metrics are called blaze_rocksdb_block_cache_usage_bytes and blaze_rocksdb_block_cache_pinned_usage_bytes.

[codecov]

Codecov Report

Merging #1008 (5b4f6ea) into develop (0c53e32) will decrease coverage by 0.01%.
The diff coverage is 93.33%.

❗ Current head 5b4f6ea differs from pull request most recent head 858180f. Consider uploading reports for the commit 858180f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1008      +/-   ##
===========================================
- Coverage    94.23%   94.22%   -0.01%     
===========================================
  Files          239      239              
  Lines        15258    15271      +13     
  Branches       372      373       +1     
===========================================
+ Hits         14378    14389      +11     
- Misses         508      509       +1     
- Partials       372      373       +1     

Impacted Files	Coverage Δ
modules/rocksdb/src/blaze/db/kv/rocksdb.clj	99.08% <66.66%> (-0.92%)	⬇️
modules/metrics/src/blaze/metrics/core.clj	100.00% <100.00%> (ø)
...odules/rocksdb/src/blaze/db/kv/rocksdb/metrics.clj	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes