feat: current settings exposed as metrics (#724)

[Adi-Goll]

This PR adds three new metrics in response to issue #724:

• max_connections
• prepared_statements_limit
• query_cache_limit

This corresponds to the following sample pgdog.toml:

[general]
openmetrics_port = 9090
prepared_statements_limit = 123
query_cache_limit = 456
default_pool_size = 70

[[databases]]
name = "mydb"
host = "127.0.0.1"
port = 5432
shard = 0

This produces the following output when metrics are displayed.

# TYPE max_connections gauge
# HELP max_connections Maximum number of server connections allowed for a pool
max_connections{user="ubuntu",database="mydb",host="127.0.0.1",port="5432",shard="0",role="primary"} 70

# TYPE prepared_statements_limit gauge
# HELP prepared_statements_limit Maximum number of prepared statements that are cached
prepared_statements_limit{user="ubuntu",database="mydb",host="127.0.0.1",port="5432",shard="0",role="primary"} 123

# TYPE query_cache_limit gauge
# HELP query_cache_limit Maximum number of queries stored in cache
query_cache_limit{user="ubuntu",database="mydb",host="127.0.0.1",port="5432",shard="0",role="primary"} 456

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 41 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/stats/pools.rs	0.00%	41 Missing ⚠️

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