Alerting: Support customizable timeout for screenshots (#60981)

This commit adds a customizable timeout for screenshots called
capture_timeout. The default value is 10 seconds, and the maximum
value is 30 seconds. This timeout should be less than the minimum
Interval of all Evaluation Groups to avoid back pressure on alert
rule evaluation.

pkg/setting/setting_unified_alerting.go

@@ -50,6 +50,8 @@ const (
 	schedulerDefaultMaxAttempts             = 3
 	schedulerDefaultLegacyMinInterval       = 1
 	screenshotsDefaultCapture               = false
+	screenshotsDefaultCaptureTimeout        = 10 * time.Second
+	screenshotsMaxCaptureTimeout            = 30 * time.Second
 	screenshotsDefaultMaxConcurrent         = 5
 	screenshotsDefaultUploadImageStorage    = false
 	// SchedulerBaseInterval base interval of the scheduler. Controls how often the scheduler fetches database for new changes as well as schedules evaluation of a rule
@@ -87,6 +89,7 @@ type UnifiedAlertingSettings struct {
 
 type UnifiedAlertingScreenshotSettings struct {
 	Capture                    bool
+	CaptureTimeout             time.Duration
 	MaxConcurrentScreenshots   int64
 	UploadExternalImageStorage bool
 }
@@ -281,6 +284,13 @@ func (cfg *Cfg) ReadUnifiedAlertingSettings(iniFile *ini.File) error {
 	uaCfgScreenshots := uaCfg.Screenshots
 
 	uaCfgScreenshots.Capture = screenshots.Key("capture").MustBool(screenshotsDefaultCapture)
+
+	captureTimeout := screenshots.Key("capture_timeout").MustDuration(screenshotsDefaultCaptureTimeout)
+	if captureTimeout > screenshotsMaxCaptureTimeout {
+		return fmt.Errorf("value of setting 'capture_timeout' cannot exceed %s", screenshotsMaxCaptureTimeout)
+	}
+	uaCfgScreenshots.CaptureTimeout = captureTimeout
+
 	uaCfgScreenshots.MaxConcurrentScreenshots = screenshots.Key("max_concurrent_screenshots").MustInt64(screenshotsDefaultMaxConcurrent)
 	uaCfgScreenshots.UploadExternalImageStorage = screenshots.Key("upload_external_image_storage").MustBool(screenshotsDefaultUploadImageStorage)
 	uaCfg.Screenshots = uaCfgScreenshots