add debug log support for the checker

This makes it possible to use this library and get a debug checker
output.

monzero.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"fmt"
+	"log/slog"
 	"time"
 )
 
@@ -19,6 +20,7 @@ type (
 		executor func(Check, context.Context) CheckResult
 		timeout  time.Duration
 		ident    string // the host identifier
+		logger   *slog.Logger
 	}
 
 	CheckerConfig struct {
@@ -42,6 +44,9 @@ type (
 		// HostIdentifier is used in notifications to point to the source of the
 		// notification.
 		HostIdentifier string
+
+		// Checker will send debug details to the logger for each command executed.
+		Logger *slog.Logger
 	}
 
 	// Check is contains the metadata to run a check and its current state.
@@ -75,6 +80,7 @@ func NewChecker(cfg CheckerConfig) (*Checker, error) {
 		executor: cfg.Executor,
 		timeout:  cfg.Timeout,
 		ident:    cfg.HostIdentifier,
+		logger:   cfg.Logger,
 	}
 	if c.executor == nil {
 		return nil, fmt.Errorf("executor must not be nil")
@@ -116,6 +122,13 @@ func (c *Checker) Next() error {
 		result.Message = fmt.Sprintf("check took longer than %s", c.timeout)
 		result.ExitCode = 2
 	}
+	c.logger.Debug(
+		"check command run",
+		"id", check.id,
+		"command", check.Command,
+		"exit code", result.ExitCode,
+		"message", result.Message,
+	)
 
 	backToOkay := false
 	if len(check.ExitCodes) == 0 && result.ExitCode == 0 {