MM-57882 Add metric for the time it takes to open the Threads list (#26983)

* MM-57882 Add metric for the time it takes to open the Threads list

* Clean up mark because the starting mark may be missing

* Pass global threads load duration to Prometheus

* Update mocks

server/einterfaces/metrics.go

@@ -112,4 +112,5 @@ type MetricsInterface interface {
 	ObserveClientChannelSwitchDuration(platform, agent string, elapsed float64)
 	ObserveClientTeamSwitchDuration(platform, agent string, elapsed float64)
 	ObserveClientRHSLoadDuration(platform, agent string, elapsed float64)
+	ObserveGlobalThreadsLoadDuration(platform, agent string, elapsed float64)
 }