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

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

server/enterprise/metrics/metrics.go

@@ -217,6 +217,7 @@ type MetricsInterfaceImpl struct {
 	ClientInteractionToNextPaint    *prometheus.HistogramVec
 	ClientCumulativeLayoutShift     *prometheus.HistogramVec
 	ClientLongTasks                 *prometheus.CounterVec
+	ClientPageLoadDuration          *prometheus.HistogramVec
 	ClientChannelSwitchDuration     *prometheus.HistogramVec
 	ClientTeamSwitchDuration        *prometheus.HistogramVec
 	ClientRHSLoadDuration           *prometheus.HistogramVec
@@ -1200,6 +1201,17 @@ func New(ps *platform.PlatformService, driver, dataSource string) *MetricsInterf
 	)
 	m.Registry.MustRegister(m.ClientLongTasks)
 
+	m.ClientPageLoadDuration = prometheus.NewHistogramVec(
+		prometheus.HistogramOpts{
+			Namespace: MetricsNamespace,
+			Subsystem: MetricsSubsystemClientsWeb,
+			Name:      "page_load",
+			Help:      "The amount of time from when the browser starts loading the web app until when the web app's load event has finished (seconds)",
+		},
+		[]string{"platform", "agent"},
+	)
+	m.Registry.MustRegister(m.ClientPageLoadDuration)
+
 	m.ClientChannelSwitchDuration = prometheus.NewHistogramVec(
 		prometheus.HistogramOpts{
 			Namespace: MetricsNamespace,
@@ -1726,6 +1738,10 @@ func (mi *MetricsInterfaceImpl) IncrementClientLongTasks(platform, agent string,
 	mi.ClientLongTasks.With(prometheus.Labels{"platform": platform, "agent": agent}).Add(inc)
 }
 
+func (mi *MetricsInterfaceImpl) ObserveClientPageLoadDuration(platform, agent string, elapsed float64) {
+	mi.ClientPageLoadDuration.With(prometheus.Labels{"platform": platform, "agent": agent}).Observe(elapsed)
+}
+
 func (mi *MetricsInterfaceImpl) ObserveClientChannelSwitchDuration(platform, agent string, elapsed float64) {
 	mi.ClientChannelSwitchDuration.With(prometheus.Labels{"platform": platform, "agent": agent}).Observe(elapsed)
 }