feat: add more metrics for StateStore

[zehaowei]

What's changed and what's your intention?

1.add iter_next_size metrics for all backends: for computing read throughput of state store
2.add existing stats of metrics for tikv & rocksdb backends
3.fix bugs of calculating metrics in memory & hummock backends

Please explain IN DETAIL what the changes are in this PR and why they are needed:

• Summarize your change (mandatory)
• How does this PR work? Need a brief introduction for the changed logic (optional)
• Describe clearly one logical change and avoid lazy messages (optional)
• Describe any limitations of the current code (optional)

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #555 (96c15bf) into main (ede9e92) will decrease coverage by 1.31%.
The diff coverage is 34.37%.

@@             Coverage Diff              @@
##               main     #555      +/-   ##
============================================
- Coverage     73.17%   71.85%   -1.32%     
+ Complexity     2709     2682      -27     
============================================
  Files           876      883       +7     
  Lines         48886    49731     +845     
  Branches       1557     1692     +135     
============================================
- Hits          35771    35733      -38     
- Misses        12304    13194     +890     
+ Partials        811      804       -7     

Flag	Coverage Δ
java	59.91% <ø> (-3.48%)	⬇️
rust	77.09% <34.37%> (-0.22%)	⬇️

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

Impacted Files	Coverage Δ
rust/storage/src/hummock/state_store.rs	0.00% <0.00%> (ø)
rust/storage/src/rocksdb_local.rs	0.00% <ø> (ø)
rust/storage/src/tikv.rs	0.00% <0.00%> (ø)
rust/storage/src/memory.rs	57.50% <31.25%> (-11.25%)	⬇️
rust/storage/src/monitor/state_store_stats.rs	98.16% <100.00%> (+0.06%)	⬆️
rust/storage/src/write_batch.rs	85.07% <100.00%> (ø)
.../risingwave/planner/rel/streaming/RwStreamAgg.java	25.00% <0.00%> (-69.45%)	⬇️
...a/com/risingwave/planner/planner/PlannerUtils.java	2.85% <0.00%> (-63.81%)	⬇️
...m/risingwave/planner/rel/physical/RwAggregate.java	46.66% <0.00%> (-43.34%)	⬇️
...isingwave/planner/rel/streaming/RwStreamChain.java	46.42% <0.00%> (-39.29%)	⬇️
... and 75 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ede9e92...96c15bf. Read the comment docs.

[Sunt-ing]

LGTM

[wcy-fdu]

LGTM for the rest