Performance metrics: Differentiate requests that are from page load (#24327)

* POC page_load_context header

* refactor to make this.pageLoadContext a private field, and relocate setTimeout code

* add pageLoadContext to `ObserveAPIEndpointDuration` method

* move to telemetry_actions

* move PageContext constant in webapp

* give server control of possible page_load_context values

* variable name change

---------

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

server/einterfaces/metrics.go

@@ -58,7 +58,7 @@ type MetricsInterface interface {
 	IncrementFilesSearchCounter()
 	ObserveFilesSearchDuration(elapsed float64)
 	ObserveStoreMethodDuration(method, success string, elapsed float64)
-	ObserveAPIEndpointDuration(endpoint, method, statusCode, originClient string, elapsed float64)
+	ObserveAPIEndpointDuration(endpoint, method, statusCode, originClient, pageLoadContext string, elapsed float64)
 	IncrementPostIndexCounter()
 	IncrementFileIndexCounter()
 	IncrementUserIndexCounter()