MM-61887: Log the userID if a metric exceeds the last histogram bucket (#29448)

We create a custom histogram metric that logs the userID
when the observed value is greater or equal to the last bucket value.

This allows us to start tracking the slowest users of a system
while at the same time not polluting the Prometheus metrics
by storing a userID for every observation.

https://mattermost.atlassian.net/browse/MM-61887

```release-note
NONE
```

server/einterfaces/metrics.go

@@ -104,16 +104,16 @@ type MetricsInterface interface {
 	IncrementNotificationNotSentCounter(notificationType model.NotificationType, notSentReason model.NotificationReason, platform string)
 	IncrementNotificationUnsupportedCounter(notificationType model.NotificationType, notSentReason model.NotificationReason, platform string)
 
-	ObserveClientTimeToFirstByte(platform, agent string, elapsed float64)
-	ObserveClientTimeToLastByte(platform, agent string, elapsed float64)
-	ObserveClientTimeToDomInteractive(platform, agent string, elapsed float64)
-	ObserveClientSplashScreenEnd(platform, agent, pageType string, elapsed float64)
+	ObserveClientTimeToFirstByte(platform, agent, userID string, elapsed float64)
+	ObserveClientTimeToLastByte(platform, agent, userID string, elapsed float64)
+	ObserveClientTimeToDomInteractive(platform, agent, userID string, elapsed float64)
+	ObserveClientSplashScreenEnd(platform, agent, pageType, userID string, elapsed float64)
 	ObserveClientFirstContentfulPaint(platform, agent string, elapsed float64)
 	ObserveClientLargestContentfulPaint(platform, agent, region string, elapsed float64)
 	ObserveClientInteractionToNextPaint(platform, agent, interaction string, elapsed float64)
 	ObserveClientCumulativeLayoutShift(platform, agent string, elapsed float64)
 	IncrementClientLongTasks(platform, agent string, inc float64)
-	ObserveClientPageLoadDuration(platform, agent string, elapsed float64)
+	ObserveClientPageLoadDuration(platform, agent, userID string, elapsed float64)
 	ObserveClientChannelSwitchDuration(platform, agent, fresh string, elapsed float64)
 	ObserveClientTeamSwitchDuration(platform, agent, fresh string, elapsed float64)
 	ObserveClientRHSLoadDuration(platform, agent string, elapsed float64)

server/einterfaces/mocks/MetricsInterface.go

@@ -328,9 +328,9 @@ func (_m *MetricsInterface) ObserveClientLargestContentfulPaint(platform string,
 	_m.Called(platform, agent, region, 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)
+// ObserveClientPageLoadDuration provides a mock function with given fields: platform, agent, userID, elapsed
+func (_m *MetricsInterface) ObserveClientPageLoadDuration(platform string, agent string, userID string, elapsed float64) {
+	_m.Called(platform, agent, userID, elapsed)
 }
 
 // ObserveClientRHSLoadDuration provides a mock function with given fields: platform, agent, elapsed
@@ -338,9 +338,9 @@ func (_m *MetricsInterface) ObserveClientRHSLoadDuration(platform string, agent
 	_m.Called(platform, agent, elapsed)
 }
 
-// ObserveClientSplashScreenEnd provides a mock function with given fields: platform, agent, pageType, elapsed
-func (_m *MetricsInterface) ObserveClientSplashScreenEnd(platform string, agent string, pageType string, elapsed float64) {
-	_m.Called(platform, agent, pageType, elapsed)
+// ObserveClientSplashScreenEnd provides a mock function with given fields: platform, agent, pageType, userID, elapsed
+func (_m *MetricsInterface) ObserveClientSplashScreenEnd(platform string, agent string, pageType string, userID string, elapsed float64) {
+	_m.Called(platform, agent, pageType, userID, elapsed)
 }
 
 // ObserveClientTeamSwitchDuration provides a mock function with given fields: platform, agent, fresh, elapsed
@@ -348,19 +348,19 @@ func (_m *MetricsInterface) ObserveClientTeamSwitchDuration(platform string, age
 	_m.Called(platform, agent, fresh, elapsed)
 }
 
-// ObserveClientTimeToDomInteractive provides a mock function with given fields: platform, agent, elapsed
-func (_m *MetricsInterface) ObserveClientTimeToDomInteractive(platform string, agent string, elapsed float64) {
-	_m.Called(platform, agent, elapsed)
+// ObserveClientTimeToDomInteractive provides a mock function with given fields: platform, agent, userID, elapsed
+func (_m *MetricsInterface) ObserveClientTimeToDomInteractive(platform string, agent string, userID string, elapsed float64) {
+	_m.Called(platform, agent, userID, elapsed)
 }
 
-// ObserveClientTimeToFirstByte provides a mock function with given fields: platform, agent, elapsed
-func (_m *MetricsInterface) ObserveClientTimeToFirstByte(platform string, agent string, elapsed float64) {
-	_m.Called(platform, agent, elapsed)
+// ObserveClientTimeToFirstByte provides a mock function with given fields: platform, agent, userID, elapsed
+func (_m *MetricsInterface) ObserveClientTimeToFirstByte(platform string, agent string, userID string, elapsed float64) {
+	_m.Called(platform, agent, userID, elapsed)
 }
 
-// ObserveClientTimeToLastByte provides a mock function with given fields: platform, agent, elapsed
-func (_m *MetricsInterface) ObserveClientTimeToLastByte(platform string, agent string, elapsed float64) {
-	_m.Called(platform, agent, elapsed)
+// ObserveClientTimeToLastByte provides a mock function with given fields: platform, agent, userID, elapsed
+func (_m *MetricsInterface) ObserveClientTimeToLastByte(platform string, agent string, userID string, elapsed float64) {
+	_m.Called(platform, agent, userID, elapsed)
 }
 
 // ObserveClusterRequestDuration provides a mock function with given fields: elapsed