Add distributed tracing to all metric instrumentation

What does this MR do?

This change adds additional opentracing instrumentation to all methods instrumented by the GitLab::Metrics instrumentation.

This vastly increases the distributed tracing coverage of the ruby application.

For example, here is an example of a trace, passing from Repository#commit, through Repository#find_commmit and eventually through to Gitaly

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• 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