feat(bin): db freelist metric

[shekhirin]

Resolves #4287

➜  reth (alexey/db-freelist-metric) ./db-tools/mdbx_stat -qff ~/Library/Application\ Support/reth/sepolia/db/mdbx.dat | grep GC
  GC: 62 pages
➜  reth (alexey/db-freelist-metric) curl -s localhost:9001 | grep '^reth_db_freelist'
reth_db_freelist 62

---

Implementation changes in freelist are due to MDBX always using 32 bits for page numbers.

LMDB page numbers: https://github.com/LMDB/lmdb/blob/c7b3cc4df6dfe8f0772fb509bdc74777667caa43/libraries/liblmdb/lmdb.h#L190-L208
MDBX page numbers:

reth/crates/storage/libmdbx-rs/mdbx-sys/libmdbx/mdbx_stat.c

Lines 2799 to 2803 in c2436a9

	/* A page number in the database.
	*
	* MDBX uses 32 bit for page numbers. This limits database
	* size up to 2^44 bytes, in case of 4K pages. */
	typedef uint32_t pgno_t;

[codecov]

Codecov Report

Merging #4346 (daa8002) into main (b473f20) will decrease coverage by 0.16%.
Report is 1 commits behind head on main.
The diff coverage is 25.00%.

Files Changed	Coverage Δ
bin/reth/src/prometheus_exporter.rs	0.00% <0.00%> (ø)
crates/storage/libmdbx-rs/src/environment.rs	80.80% <100.00%> (+0.37%)	⬆️

... and 11 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.72% <25.00%> (-0.04%)	⬇️
unit-tests	63.73% <0.00%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	26.01% <0.00%> (-0.02%)	⬇️
blockchain tree	82.56% <ø> (ø)
pipeline	90.07% <ø> (ø)
storage (db)	74.72% <100.00%> (+<0.01%)	⬆️
trie	94.88% <ø> (+0.03%)	⬆️
txpool	47.94% <ø> (-0.51%)	⬇️
networking	77.54% <ø> (+0.06%)	⬆️
rpc	57.99% <ø> (-0.82%)	⬇️
consensus	63.53% <ø> (ø)
revm	31.97% <ø> (ø)
payload builder	6.78% <ø> (ø)
primitives	86.36% <ø> (ø)