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

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

server/einterfaces/mocks/MetricsInterface.go

@@ -323,6 +323,11 @@ func (_m *MetricsInterface) ObserveClientLargestContentfulPaint(platform string,
 	_m.Called(platform, agent, elapsed)
 }
 
+// ObserveClientPageLoadDuration provides a mock function with given fields: platform, agent, elapsed
+func (_m *MetricsInterface) ObserveClientPageLoadDuration(platform string, agent string, elapsed float64) {
+	_m.Called(platform, agent, elapsed)
+}
+
 // ObserveClientRHSLoadDuration provides a mock function with given fields: platform, agent, elapsed
 func (_m *MetricsInterface) ObserveClientRHSLoadDuration(platform string, agent string, elapsed float64) {
 	_m.Called(platform, agent, elapsed)