pgxpool: enable metrics (#5653)

## Summary

Enables metrics for the `pgxpool` that is used by the PostgreSQL
databroker backend.
Metrics are updated at 1s interval. 
Will add the following metric output in the regular Prometheus
`/metrics` endpoint:

```
# HELP pomerium_pgxpool_acquire_duration_nanoseconds_total Total duration of all successful acquires from the pool in nanoseconds.
# TYPE pomerium_pgxpool_acquire_duration_nanoseconds_total counter
pomerium_pgxpool_acquire_duration_nanoseconds_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5.1058702e+07
# HELP pomerium_pgxpool_acquired_connections Number of currently acquired connections in the pool.
# TYPE pomerium_pgxpool_acquired_connections gauge
pomerium_pgxpool_acquired_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_acquires_total Cumulative count of successful acquires from the pool.
# TYPE pomerium_pgxpool_acquires_total counter
pomerium_pgxpool_acquires_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 91
# HELP pomerium_pgxpool_canceled_acquires_total Cumulative count of acquires from the pool that were canceled by a context.
# TYPE pomerium_pgxpool_canceled_acquires_total counter
pomerium_pgxpool_canceled_acquires_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_constructing_connections_milliseconds Number of connections with construction in progress in the pool.
# TYPE pomerium_pgxpool_constructing_connections_milliseconds gauge
pomerium_pgxpool_constructing_connections_milliseconds{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_empty_acquire_total Cumulative count of successful acquires from the pool that waited for a resource to be released or constructed because the pool was empty.
# TYPE pomerium_pgxpool_empty_acquire_total counter
pomerium_pgxpool_empty_acquire_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 6
# HELP pomerium_pgxpool_idle_connections Number of currently idle connections in the pool.
# TYPE pomerium_pgxpool_idle_connections gauge
pomerium_pgxpool_idle_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5
# HELP pomerium_pgxpool_max_connections Maximum size of the pool.
# TYPE pomerium_pgxpool_max_connections gauge
pomerium_pgxpool_max_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 10
# HELP pomerium_pgxpool_max_idle_destroys_total Cumulative count of connections destroyed because they exceeded MaxConnectionsIdleTime.
# TYPE pomerium_pgxpool_max_idle_destroys_total counter
pomerium_pgxpool_max_idle_destroys_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_max_lifetime_destroys_total Cumulative count of connections destroyed because they exceeded MaxConnectionsLifetime.
# TYPE pomerium_pgxpool_max_lifetime_destroys_total counter
pomerium_pgxpool_max_lifetime_destroys_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 0
# HELP pomerium_pgxpool_new_connections_total Cumulative count of new connections opened.
# TYPE pomerium_pgxpool_new_connections_total counter
pomerium_pgxpool_new_connections_total{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 6
# HELP pomerium_pgxpool_total_connections Total number of resources currently in the pool. The value is the sum of ConstructingConnections, AcquiredConnections, and IdleConnections.
# TYPE pomerium_pgxpool_total_connections gauge
pomerium_pgxpool_total_connections{db_client_connection_pool_name="localhost:5432/pomerium",db_system="postgresql",otel_scope_name="github.com/exaring/otelpgx",otel_scope_version="v0.9.1",hostname="MacBookPro"} 5
```

## Related issues

<!-- For example...
- #159
-->

## User Explanation

<!-- How would you explain this change to the user? If this
change doesn't create any user-facing changes, you can leave
this blank. If filled out, add the `docs` label -->

## Checklist

- [x] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

pkg/storage/postgres/backend.go

@@ -361,24 +361,29 @@ func (backend *Backend) init(ctx context.Context) (serverVersion uint64, pool *p
 
 	pool, err = pgxpool.NewWithConfig(context.Background(), config)
 	if err != nil {
-		return serverVersion, nil, err
+		return serverVersion, nil, fmt.Errorf("error creating pgxpool: %w", err)
+	}
+
+	err = otelpgx.RecordStats(pool)
+	if err != nil {
+		return serverVersion, nil, fmt.Errorf("error recording stats: %w", err)
 	}
 
 	tx, err := pool.Begin(ctx)
 	if err != nil {
-		return serverVersion, nil, err
+		return serverVersion, nil, fmt.Errorf("error starting transaction: %w", err)
 	}
 
 	serverVersion, err = migrate(ctx, tx)
 	if err != nil {
 		_ = tx.Rollback(ctx)
-		return serverVersion, nil, err
+		return serverVersion, nil, fmt.Errorf("error running migrations: %w", err)
 	}
 
 	err = tx.Commit(ctx)
 	if err != nil {
 		_ = tx.Rollback(ctx)
-		return serverVersion, nil, err
+		return serverVersion, nil, fmt.Errorf("error committing transaction: %w", err)
 	}
 
 	backend.serverVersion = serverVersion