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>

webapp/platform/client/src/client4.ts

@@ -216,6 +216,14 @@ export default class Client4 {
         this.defaultHeaders['Accept-Language'] = locale;
     }
 
+    setHeader(header: string, value: string) {
+        this.defaultHeaders[header] = value;
+    }
+
+    removeHeader(header: string) {
+        delete this.defaultHeaders[header];
+    }
+
     setEnableLogging(enable: boolean) {
         this.enableLogging = enable;
     }