Conditionally initialize the global opentracing tracer

What does this MR do?

Requires https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24422

This is an attempt to slim https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21280 down by breaking the functionality out.

This change will instantiate an OpenTracing tracer and configure it as the global tracer when the GITLAB_TRACING environment variable is configured. GITLAB_TRACING takes a "connection string"-like value, encapsulating the driver (eg jaeger, etc) and options for the driver. Since each service, whether it's written in Ruby or Golang, uses the same connection-string, it should be very easy to configure all services in a cluster, or even a single development machine to be setup to use tracing.

Note that this change does not include instrumentation or propagation changes as this is a way of breaking a previous larger change into components. The instrumentation and propagation changes will follow in separate patches.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer