feat: handle span compression with new service_target_* fields

[kruskall]

Update compression logic to use service target fields and stop relying on resource internally.

The updated logic is compliant with the spec on span compression.
See https://github.com/elastic/apm/blob/main/specs/agents/handling-huge-traces/tracing-spans-compress.md#consecutive-same-kind-compression-strategy

Closes #1292

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-11-28T10:21:23.346+0000

• Duration: 59 min 37 sec

Test stats 🧪

Test	Results
Failed	0
Passed	8688
Skipped	215
Total	8903

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark test.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[apmmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (60/60)	💚
Files	99.355% (154/155)	👍
Classes	96.317% (340/353)	👍
Methods	90.349% (983/1088)	👍
Lines	82.252% (11456/13928)	👍 0.073
Conditionals	100.0% (0/0)	💚

[axw]

I'm surprised that no tests have changed. Do we have any tests for "same kind" compression with target service?

[simitt]

@kruskall have you had time to follow up with the question regarding the tests?

[kruskall]

I'm surprised that no tests have changed. Do we have any tests for "same kind" compression with target service?

Sorry for the late reply. I'm not sure what should change. We have tests to verify "same kind" compression is working as intended (https://github.com/elastic/apm-agent-go/blob/main/span_test.go#L536-L656) but they still pass since there is no "change" in behaviour.
Service target fields are fully supported and inferred when possible, same as the old destination service. Moving to the new fields would not change the "same kind" relantionship between spans created the same way.

[axw]

It seems to me we have a (pre-existing) gap in our testing: AFAICS the "same kind" compression tests don't cover the code path where the span type and subtype are the same, but the target service name/type differ. I expected we would have a test for this, and that we should have changed the test to set service target rather than the deprecated destination fields.

We can merge this as is, but it would be good to improve the tests.

[kruskall]

It seems to me we have a (pre-existing) gap in our testing: AFAICS the "same kind" compression tests don't cover the code path where the span type and subtype are the same, but the target service name/type differ. I expected we would have a test for this, and that we should have changed the test to set service target rather than the deprecated destination fields.

We can merge this as is, but it would be good to improve the tests.

Good point! I think we can add it here since it's related to the change and it's fairly small. I've added a test case, let me know what you think! 👍

[axw]

LGTM, but I think we could simplify the test a little bit by not relying on inference.

[axw]

Looks great, thank you :)

[marclop]

Verified with 8.7.0 BC9 and the latest main (332875e):

package main

import (
	"fmt"
	"time"

	"go.elastic.co/apm/v2"
)

func main() {
	tracer := apm.DefaultTracer()
	tracer.SetSpanCompressionEnabled(true)
	tracer.SetExitSpanMinDuration(0)
	currentTime := time.Now()
	tx := tracer.StartTransactionOptions("8.7.0", "testplan", apm.TransactionOptions{
		Start: currentTime,
	})
	currentTime.Add(
		newSpan(tx, currentTime, "SELECT * FROM users", "mysql", "db", "myhost"),
	)
	currentTime.Add(
		newSpan(tx, currentTime, "SELECT * FROM users", "mysql", "db", "myhost"),
	)
	currentTime.Add(
		newSpan(tx, currentTime, "SELECT * FROM orders", "mysql", "db", "anotherhost"),
	)
	tx.End()
	tracer.Flush(nil)
	fmt.Printf("%+v\n", tracer.Stats())
}

func newSpan(tx *apm.Transaction, start time.Time, name, t, targetType, targetName string) time.Duration {
	span := tx.StartSpanOptions(name, t, apm.SpanOptions{
		ExitSpan: true, Start: start,
	})
	span.Duration = 2 * time.Microsecond
	span.Context.SetServiceTarget(apm.ServiceTargetSpanContext{
		Type: targetType,
		Name: targetName,
	})
	span.End()
	return span.Duration
}