Adds BrowserTracing to Sentry

What does this MR do and why?

This change adds a missing BrowserTracing integration configuration.

Initially, I assumed this was part of the default integrations, so I didn't include it. This addition allows us to do performance tracing on the browser.

This is a follow up to !130713 (merged), which was not complete, my bad. 😓

Screenshots or screen recordings

We can see the (big) payload that's being sent, unlike the usual session tracking it sends a lot more data:

See the result in sentry:

How to set up and validate locally

1. Testing on GDK, requires disabling CSP on your browser (I temporarily used https://chrome.google.com/webstore/detail/disable-content-security/ieelmcmcagommplceebfedjlakkhpden, caution, don't keep this extension installed)
2. Get the clientside DSN at https://new-sentry.gitlab.net/settings/gitlab/projects/gitlabcom-clientside/keys/, it looks like https://XXXX@new-sentry.gitlab.net/4
3. Enable Sentry at Admin -> Metrics and profiling http://gdk.test:3000/admin/application_settings/metrics_and_profiling#js-sentry-settings, with the setting you see here, plus the DSN:
   • 
4. We use a 1,0 in Clientside traces sample rate so our tracing report is always sent.
5. Refresh the page, that should already send a request to Sentry.
6. Clean up! Disable Sentry our local.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #339332 (closed)