MM-58359 Add page_load to Prometheus metrics (#27159)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/einterfaces/metrics.go

@@ -109,6 +109,7 @@ type MetricsInterface interface {
 	ObserveClientInteractionToNextPaint(platform, agent string, elapsed float64)
 	ObserveClientCumulativeLayoutShift(platform, agent string, elapsed float64)
 	IncrementClientLongTasks(platform, agent string, inc float64)
+	ObserveClientPageLoadDuration(platform, agent string, elapsed float64)
 	ObserveClientChannelSwitchDuration(platform, agent string, elapsed float64)
 	ObserveClientTeamSwitchDuration(platform, agent string, elapsed float64)
 	ObserveClientRHSLoadDuration(platform, agent string, elapsed float64)