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/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