Intercept OTEL span links through `.addLink`

[jsumners-nr]

While implementing #3474, I missed that the Span object has an .addLink method to allow adding of links during the lifetime of the span. The implementation for #3474 handles the links present at creation of the span. While it is recommended to only add links at creation, the reality is that links can be added subsequent to creation and we need to account for those.

We need to update our span onStart method to intercept the .addLink method on the span to handle in-flight links. It will likely look something like:

onStart(span) {
  span[otelSynthesis] = this.synthesizer.synthesize(span)
  const { segment } = span[otelSynthesis]
  
  const addLink = span.addLink
  span.addLink = function nrAddLink(link) {
    const nrLink = new SpanLink({ link, context: span.spanContext(), timestamp: span.startTime })
    segment.addSpanLink(nrLink)
    addLink.call(span, link)
  }

  const addLinks = span.addLinks
  span.addLinks = function nrAddLinks(links) {
    for (const link of links) { span.addLink(link) }
  }
}

node-newrelic/lib/otel/traces/span-processor.js

Lines 42 to 44 in ae0074b

	onStart(span) {
	span[otelSynthesis] = this.synthesizer.synthesize(span)
	}

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-500659

[jsumners-nr]

An alternate solution could be to do all of the work in the onEnd, including migrating our current code there.