[web] Add startup metrics (#25277)

* [web] refactor startup sequence

The primary objective here is to call loadGlobalBlobs() only once.
But to get there, we need to reorder things and add extra try/catch
sections to ensure we are not letting the global uncaughtException
handler catch startup errors.

Co-authored-by: Antoine Clausse <antoine.clausse@overleaf.com>

* [web] add metrics for startup steps

Co-authored-by: Antoine Clausse <antoine.clausse@overleaf.com>

---------

Co-authored-by: Jakob Ackermann <jakob.ackermann@overleaf.com>
GitOrigin-RevId: c73edea02516e919d55b896588dcd1862835fedf

libraries/metrics/initialize.js

@@ -5,6 +5,8 @@
  * before any other module to support code instrumentation.
  */
 
+const metricsModuleImportStartTime = performance.now()
+
 const APP_NAME = process.env.METRICS_APP_NAME || 'unknown'
 const BUILD_VERSION = process.env.BUILD_VERSION
 const ENABLE_PROFILE_AGENT = process.env.ENABLE_PROFILE_AGENT === 'true'
@@ -103,3 +105,5 @@ function recordProcessStart() {
   const metrics = require('.')
   metrics.inc('process_startup')
 }
+
+module.exports = { metricsModuleImportStartTime }