## Description
- add configurable DB pool size
- monitor DB pool, the total size and idle conn
## Test Plan
local run and saw metrics on localhost
```
indexer_idle_db_conn{indexer_fullnode="ord-mnt-rpcbig-06.mainnet.sui"} 100
indexer_db_conn_pool_size{indexer_fullnode="ord-mnt-rpcbig-06.mainnet.sui"} 100
```
and logs
```
2023-07-29T20:50:37.370315Z INFO sui_indexer: DB connection pool size: 100, with idle conn: 100.
```
---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.
### Type of Change (Check all that apply)
- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration
### Release notes
