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

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

server/channels/app/metrics.go

@@ -36,6 +36,8 @@ func (a *App) RegisterPerformanceReport(rctx request.CTX, report *model.Performa
 			a.Metrics().ObserveClientInteractionToNextPaint(commonLabels["platform"], commonLabels["agent"], h.Value/1000)
 		case model.ClientCumulativeLayoutShift:
 			a.Metrics().ObserveClientCumulativeLayoutShift(commonLabels["platform"], commonLabels["agent"], h.Value)
+		case model.ClientPageLoadDuration:
+			a.Metrics().ObserveClientPageLoadDuration(commonLabels["platform"], commonLabels["agent"], h.Value/1000)
 		case model.ClientChannelSwitchDuration:
 			a.Metrics().ObserveClientChannelSwitchDuration(commonLabels["platform"], commonLabels["agent"], h.Value/1000)
 		case model.ClientTeamSwitchDuration: