Install Jaeger by default in all Sourcegraph deployment options

Created by: beyang

Top-line goal: Boost use of Jaeger at customers to close to 100%. Sub-goals:

• Update installation instructions to include Jaeger for all 3 deployment methods:
  • k8s Sourcegraph (https://github.com/sourcegraph/deploy-sourcegraph/pull/559)
  • Single Docker image
  • :running_man: Docker Compose
• Turn Jaeger on/off using config watch (no restart required)
• Site config field that enables selective sampling triggered by URL parameter: jaeger.sampling: {selective,comprehensive} (necessary to address https://github.com/sourcegraph/sourcegraph/issues/5363)
• Deprecate useJaeger site config option
• Jaeger collection should be toggleable without needing to restart sourcegraph-frontend service
• Add docs for changing Jaeger sampling strategy (https://www.jaegertracing.io/docs/1.17/sampling/)
• Migration docs for existing customers (need to cover installation instructions and possible site config migration)
• Make it easier to report slow queries by linking to Jaeger info. E.g., slow searches should prompt to report the issue with a link to the Jaeger span.

Follow-up tasks

• From @keegan: I think using @uwedeportivo's work around admin proxying to include Jaeger would be very useful here. Then updating x-trace header to link via the admin proxy. Moved to: https://github.com/sourcegraph/sourcegraph/issues/10048