Add metric to track the display of the install browser extension popover

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/26319.

Description

This PR:

• Adds BrowserExtensionPopupOpened event when Install Browser Extension Popover is shown

How to test

• Run web and make popover to display locally
  • Update sg.config.yml->web-standalone-http->SOURCEGRAPH_API_URL to https://sourcegraph.com. This is just to avoid signing up for k8s.sgdev.org
  • Update https://github.com/sourcegraph/sourcegraph/blob/ab7de62d80cb5b1e6d1fcde4d13757dbada37f60/client/web/src/repo/RepoContainer.tsx#L339 to just true to avoid uninstalling bext and making all conditions work in order to display a popover
  • Start web sg run web-standalone-http
• Open any repo page home page, ex: http://localhost:3080/github.com/sourcegraph/sourcegraph
• Click on the "View on Github" icon and check in devTools->Network that event BrowserExtensionPopupOpened was sent

Note: that events triggered within 1 sec are joined together in the network and sent as array of events

Before merging

• Test on different code hosts (if applicable)

  • GitHub
  • Gitlab
  • GitHub Enterprise
  • Refined GitHub
  • Phabricator
  • Phabricator integration
  • Bitbucket
  • Bitbucket integration

• Test on different browsers (if applicable)

  • Chrome
  • Firefox
  • Safari

• Add change log message to client/browser/CHANGELOG.md, under "Unreleased" section. (if applicable)