Derive outcome from the HTTP status code when available

[jalvz]

Motivation/summary

Avoid big empty charts in Kibana for error rates when user have ~old agents

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:

• documentation
• logging (add log lines, choose appropriate log selector, etc.)
• metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

Related issues

Closes #4164

[jalvz]

Missing spec update (coming soon)

Spec: elastic/apm#338

[apmmachine]

💚 Build Succeeded

Expand to view the summary

Build stats

• Build Cause: [Pull request #4165 updated]

• Start Time: 2020-09-08T09:16:51.111+0000

• Duration: 44 min 47 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3273
Skipped	145
Total	3418

Steps errors

Expand to view the steps failures

• Name: Compress

  • Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

  • Duration: 0 min 0 sec

  • Start Time: 2020-09-08T09:32:57.735+0000

  • log

• Name: Compress

  • Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

  • Duration: 0 min 0 sec

  • Start Time: 2020-09-08T09:45:52.445+0000

  • log

• Name: Test Sync

  • Description: ./script/jenkins/sync.sh

  • Duration: 3 min 50 sec

  • Start Time: 2020-09-08T09:26:59.513+0000

  • log

[axw]

Looks good! Just a few minor things.

[axw]

Tested BC1 using the Go agent v1.8.0, which does not send outcome.

package main

import (
        "net/http"

        "go.elastic.co/apm"
)

func main() {
        tracer := apm.DefaultTracer
        tx := tracer.StartTransaction("name", "type")
        span := tx.StartSpan("name", "type", nil)
        req, _ := http.NewRequest("GET", "http://testing.invalid", nil)
        span.Context.SetHTTPRequest(req)
        span.Context.SetHTTPStatusCode(200)
        span.End()
        tx.Context.SetHTTPStatusCode(400)
        tx.End()
        tracer.Flush(nil)
}

As expected:

• With transaction's HTTP status code set to 400, event.outcome is "success"
• With transaction's HTTP status code set to 500, event.outcome is "failure"
• With span's HTTP status code set to 200, event.outcome is "success"
• With span's HTTP status code set to 400, event.outcome is "failure"