feat: exit spans, infer destination.service.resource

[trentm]

This adds the explicit concept of "exit spans" -- spans the represent an outgoing call to a downstream service. An exit span doesn't have child spans. (Technically it can have spans of the same type+subtype, but this agent doesn't have any cases of that.) An exit span is explicitly marked at creation time by setting the exitSpan option to true. https://github.com/elastic/apm/blob/master/specs/agents/tracing-spans.md#exit-spans

For exit spans the 'span.context.destination.service.resource' is automatically inferred from other context. This can be overriden with the new span.setDestinationService(resource). https://github.com/elastic/apm/blob/master/specs/agents/tracing-spans-destination.md

Fixes: #2103

Checklist

• [ ] Implement code
  • [ ] inferred http resource from URL (avoid re-parsing)
  • [x] no-child-span handling
  • [ ] perhaps include http context in ES and aws-sdk spans as part of this PR
  • [ ] this likely depends on parent/child run-context fixes from #2430
• [ ] Add tests
• [x] Update TypeScript typings
• [x] Update documentation
• [ ] Add CHANGELOG.asciidoc entry
• [x] Commit message follows commit guidelines

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• Start Time: 2021-11-29T15:24:48.700+0000

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[trentm]

This was obsoleted by #2552 that added exit spans and #2882 that added service.target.* fields, and the inference of destination.service.resource from service.target.* and other span fields.