[APM] Support span links

[felixbarny]

Summary

We have several use cases where Span links could come in handy. To understand what span links are, let's look at the use-cases we want to solve:

• When a single transaction represents the batch processing of several messages, span links help to link back to the traces that have produced the messages
• Another use case related to messaging is whenever there is a span that represents message polling. If the returned message has a traceparent header, we can infer the sender. However, since it already has a parent, we currently have no way to link to the sender.
• The traceparent header becomes more and more pervasive. There's a good chance that applications get traceparent headers from outside a trust boundary. In this case, we want to restart the trace (creating a different trace id with its own sampling decision) and link to the originating trace.

Proposed solution

The desired solution is to show that span links are available on the individual transaction and span rows in the trace waterfall. That will enable the user to open the detail flyouts and learn what links are available for the individual transaction or span and navigate to the relevant trace that is linked.

There can be 1 or more span links available per transaction and spans, so we need to ensure that the design supports this.

• Show span links are available for the individual transactions or spans in the Trace waterfall
• Show span links in the transaction or span detail view

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)