telemetrygateway: add external URL to instance identifier

[bobheadxi]

This change adds the external URL to the exported events payload. We send it once at the beginning of the stream as part of the stream metadata, and in Telemetry Gateway this gets joined onto each event payload, per request: https://sourcegraph.slack.com/archives/C05BGNBEPKL/p1702584292491359?thread_ts=1702583517.559349&cid=C05BGNBEPKL

Test plan

Unit tests

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[sourcegraph-release-guild-bot]

The backport to 5.2 failed at https://github.com/sourcegraph/sourcegraph/actions/runs/7225925130:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-5.2 5.2
# Navigate to the new working tree
cd .worktrees/backport-5.2
# Create a new branch
git switch --create backport-59014-to-5.2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 172cbe3711fd2e0e2c75ebb88c2153526028e4e1
# Push it to GitHub
git push --set-upstream origin backport-59014-to-5.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-5.2

If you encouter conflict, first resolve the conflict and stage all files, then run the commands below:

git cherry-pick --continue
# Push it to GitHub
git push --set-upstream origin backport-59014-to-5.2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-5.2

• Follow above instructions to backport the commit.
• Create a pull request where the base branch is 5.2 and the compare/head branch is backport-59014-to-5.2., click here to create the pull request.
• Make sure to tag @sourcegraph/release-guild in the pull request description.
• Once the backport pull request is created, kindly remove the release-blocker from this pull request.