Add support for span links

[axw]

Motivation/summary

This PR adds support for span links to our data model, enabling a span/transaction to be associated with zero or more other spans/transactions. We use span.links and span.links.span.id regardless of whether the linked entities are spans or transactions.

The PR also adds support for capturing OpenTelemetry span links.

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
  - [ ] Documentation has been updated

How to test these changes

1. Using an OpenTelemetry SDK, send a span with links to APM Server.
2. Check that the trace ID and span ID of the link are stored. Any attributes or trace-state associated with the link should not be stored.
3. Ensure the trace can be find when searching traces-apm* and querying by span.links.trace.id

Related issues

Closes #7171

[apmmachine]

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Build stats

• Start Time: 2022-02-16T10:30:58.673+0000

• Duration: 64 min 27 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5644
Skipped	19
Total	5663

🤖 GitHub comments

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[felixbarny]

LGTM, and easy to expand with span link attributes/labels later on.

[stuartnelson3]

confirmed with 4575383

1. start kibana/elasticsearch
2. start apm-server
3. send traces with the following program:

package main

import (
	"context"
	"log"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc"
	"go.opentelemetry.io/otel/propagation"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	"go.opentelemetry.io/otel/trace"
)

var tracer = otel.Tracer("tracer")

func main() {
	tp := initTracer()
	defer func() {
		if err := tp.Shutdown(context.Background()); err != nil {
			log.Printf("Error shutting down tracer provider: %v", err)
		}
	}()

	ctx, span := tracer.Start(context.Background(), "span")
	defer span.End()
	link := trace.LinkFromContext(ctx, attribute.Int("attr", 1))
	_, span2 := tracer.Start(ctx, "linkedSpan", trace.WithLinks(link))
	defer span2.End()
}

func initTracer() *sdktrace.TracerProvider {
	ctx := context.Background()
	// Create the OTLP exporter that
	// will receive the telemetry data
	endpoint := "localhost:8200"
	exporter, err := otlptracegrpc.New(ctx,
		otlptracegrpc.WithInsecure(),
		otlptracegrpc.WithEndpoint(endpoint),
	)
	if err != nil {
		log.Fatal(err)
	}
	tp := sdktrace.NewTracerProvider(
		sdktrace.WithSampler(sdktrace.AlwaysSample()),
		sdktrace.WithBatcher(exporter),
	)
	otel.SetTracerProvider(tp)
	otel.SetTextMapPropagator(
		propagation.NewCompositeTextMapPropagator(
			propagation.TraceContext{},
			propagation.Baggage{},
		),
	)
	return tp
}

4. go to kibana console and verify both traces are there, and one has span links

GET traces-apm-default/_search

5. get the span.links.trace.id and check you can query with it

GET traces-apm-default/_search
{
  "query": {
    "match": {
      "span.links.trace.id": "d1e6e6a15b5f93eb18c70f237f9ce3b3"
    }
  }
}