feat: expose connection pool stats

[turip]

Overview

This patch exposes the connection pool statistics via opentelemetry, so that we see for all the workers the distribution of idle/acquired etc. connections.

Example metrics:

# HELP pgxpool_acquire_count_total The cumulative count of successful acquires from the pool.
# TYPE pgxpool_acquire_count_total counter
pgxpool_acquire_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 4
# HELP pgxpool_acquire_duration_avg_milliseconds The average duration of all successful acquires from the pool in ms.
# TYPE pgxpool_acquire_duration_avg_milliseconds gauge
pgxpool_acquire_duration_avg_milliseconds{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 2
# HELP pgxpool_acquire_duration_milliseconds The total duration of all successful acquires from the pool in ms.
# TYPE pgxpool_acquire_duration_milliseconds gauge
pgxpool_acquire_duration_milliseconds{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 10
# HELP pgxpool_acquired_conns The number of currently acquired connections in the pool.
# TYPE pgxpool_acquired_conns gauge
pgxpool_acquired_conns{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 0
# HELP pgxpool_canceled_acquire_count_total The cumulative count of acquires from the pool that were canceled by a context.
# TYPE pgxpool_canceled_acquire_count_total counter
pgxpool_canceled_acquire_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 0
# HELP pgxpool_constructing_conns The number of conns with construction in progress in the pool.
# TYPE pgxpool_constructing_conns gauge
pgxpool_constructing_conns{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 0
# HELP pgxpool_empty_acquire_count_total The cumulative count of successful acquires from the pool that waited for a resource to be released or constructed because the pool was empty.
# TYPE pgxpool_empty_acquire_count_total counter
pgxpool_empty_acquire_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 1
# HELP pgxpool_idle_conns The number of currently idle conns in the pool.
# TYPE pgxpool_idle_conns gauge
pgxpool_idle_conns{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 1
# HELP pgxpool_max_conns The maximum size of the pool.
# TYPE pgxpool_max_conns gauge
pgxpool_max_conns{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 16
# HELP pgxpool_max_idle_destroy_count_total The cumulative count of connections closed due to reaching their maximum idle time (MaxConnIdleTime).
# TYPE pgxpool_max_idle_destroy_count_total counter
pgxpool_max_idle_destroy_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 0
# HELP pgxpool_max_lifetime_destroy_count_total The cumulative count of connections closed due to reaching their maximum lifetime (MaxConnLifetime).
# TYPE pgxpool_max_lifetime_destroy_count_total counter
pgxpool_max_lifetime_destroy_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 0
# HELP pgxpool_new_conns_count_total The cumulative count of new connections opened.
# TYPE pgxpool_new_conns_count_total counter
pgxpool_new_conns_count_total{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 1
# HELP pgxpool_total_conns The total number of resources currently in the pool. The value is the sum of ConstructingConns, AcquiredConns, and IdleConns.
# TYPE pgxpool_total_conns gauge
pgxpool_total_conns{otel_scope_name="openmeter.io/backend",otel_scope_version=""} 1

Notes for reviewer

@sagikazarmark if this works well enough we can split this into a separate package.