Add missing import in observability generator

Created by: lguychard

Fixe the following error when running dev/start:

./src-observability-generator.go:246:23: undefined: rand
src-observability-generator.go:2: running "go": exit status 2

I believe this would also fix the build on master: https://buildkite.com/sourcegraph/sourcegraph/builds/60393#cd0a50a0-91a4-43a5-b5c5-785ee1a9ae11/608-617