fix(database): correct metrics and improve error handling

- Fix metrics double-counting: track deltas for WaitCount/WaitDuration
  instead of adding cumulative values each tick
- Replace fmt.Printf with structured logging in pool monitor
- Add PoolOptions validation (MaxConns > 0, MinConns >= 0)
- Warn when DATABASE_URI overrides non-default PoolOptions
- Improve findAndParseConfig to report all tried files and errors
- Remove dead code in pgdb/config.go (unreachable host default)
- Fix errcheck lint issues for file.Close() calls
- Add context parameter to OpenDBMonitor() (breaking change)

database/pgdb/pool.go

@@ -2,12 +2,14 @@ package pgdb
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
 	"go.ntppool.org/common/database"
+	"go.ntppool.org/common/logger"
 	"gopkg.in/yaml.v3"
 )
 
@@ -61,6 +63,19 @@ func DefaultPoolOptions() PoolOptions {
 // can be specified in the URI query string and PoolOptions are ignored.
 // When using config files, PoolOptions are applied.
 func OpenPool(ctx context.Context, options PoolOptions) (*pgxpool.Pool, error) {
+	log := logger.FromContext(ctx)
+
+	// Validate PoolOptions
+	if options.MaxConns <= 0 {
+		return nil, fmt.Errorf("pgdb: MaxConns must be positive, got: %d", options.MaxConns)
+	}
+	if options.MinConns < 0 {
+		return nil, fmt.Errorf("pgdb: MinConns must be non-negative, got: %d", options.MinConns)
+	}
+	if options.MinConns > options.MaxConns {
+		return nil, fmt.Errorf("pgdb: MinConns (%d) cannot exceed MaxConns (%d)", options.MinConns, options.MaxConns)
+	}
+
 	var poolConfig *pgxpool.Config
 	var err error
 
@@ -70,7 +85,15 @@ func OpenPool(ctx context.Context, options PoolOptions) (*pgxpool.Pool, error) {
 		if err != nil {
 			return nil, fmt.Errorf("failed to parse DATABASE_URI: %w", err)
 		}
-		// Pool settings from URI are used; PoolOptions ignored
+
+		// Log when PoolOptions differ from defaults (they will be ignored)
+		defaults := DefaultPoolOptions()
+		if options.MaxConns != defaults.MaxConns || options.MinConns != defaults.MinConns {
+			log.WarnContext(ctx, "DATABASE_URI is set; PoolOptions are ignored (use URI query parameters for pool settings)",
+				"ignored_max_conns", options.MaxConns,
+				"ignored_min_conns", options.MinConns,
+			)
+		}
 	} else {
 		// Fall back to config file approach
 		pgCfg, err := findAndParseConfig(options.ConfigFiles)
@@ -99,7 +122,7 @@ func OpenPool(ctx context.Context, options PoolOptions) (*pgxpool.Pool, error) {
 
 	// Test the connection
 	if err := pool.Ping(ctx); err != nil {
-		pool.Close()
+		pool.Close() // pgxpool.Pool.Close() doesn't return an error
 		return nil, fmt.Errorf("failed to ping database: %w", err)
 	}
 
@@ -116,35 +139,33 @@ func OpenPoolWithConfigFile(ctx context.Context, configFile string) (*pgxpool.Po
 
 // findAndParseConfig searches for and parses the first existing config file
 func findAndParseConfig(configFiles []string) (*database.PostgresConfig, error) {
-	var firstErr error
+	var errs []error
+	var triedFiles []string
 
 	for _, configFile := range configFiles {
 		if configFile == "" {
 			continue
 		}
+		triedFiles = append(triedFiles, configFile)
 
 		// Check if file exists
 		if _, err := os.Stat(configFile); err != nil {
-			if firstErr == nil {
-				firstErr = err
-			}
+			errs = append(errs, fmt.Errorf("%s: %w", configFile, err))
 			continue
 		}
 
 		// Try to read and parse the file
 		pgCfg, err := parseConfigFile(configFile)
 		if err != nil {
-			if firstErr == nil {
-				firstErr = err
-			}
+			errs = append(errs, fmt.Errorf("%s: %w", configFile, err))
 			continue
 		}
 
 		return pgCfg, nil
 	}
 
-	if firstErr != nil {
-		return nil, fmt.Errorf("no config file found: %w", firstErr)
+	if len(errs) > 0 {
+		return nil, fmt.Errorf("no valid config file found (tried: %v): %w", triedFiles, errors.Join(errs...))
 	}
 	return nil, fmt.Errorf("no valid config files provided")
 }
@@ -155,7 +176,7 @@ func parseConfigFile(configFile string) (*database.PostgresConfig, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to open config file: %w", err)
 	}
-	defer file.Close()
+	defer func() { _ = file.Close() }()
 
 	dec := yaml.NewDecoder(file)
 	cfg := database.Config{}