error: Record transaction.name when available

[marclop]

Description

Adds a new transaction.name field which is recorded when a transaction
is set for the error.

Related issues

Closes #1154

[ghost]

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Build stats

• Start Time: 2021-12-22T09:10:12.825+0000

• Duration: 33 min 3 sec

• Commit: 5a081c1

Test stats 🧪

Test	Results
Failed	0
Passed	11976
Skipped	268
Total	12244

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[axw]

Verified using go.elastic.co/apm/v2 v2.0.0-20220209134054-640a916049c9 (current main).

package main

import (
        "context"
        "errors"

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

func main() {
        tracer := apm.DefaultTracer()
        defer tracer.Flush(nil)

        tracer.NewError(errors.New("err1")).Send()

        tx := tracer.StartTransaction("name", "type")
        ctx := apm.ContextWithTransaction(context.Background(), tx)
        apm.CaptureError(ctx, errors.New("err2")).Send()
        tx.Name = "new_name"
        tx.End()
}

• The first error has no transaction.name field at all, as expected.
• The second error has transaction.name: name -- the transaction name at the time the error was captured.
• The transaction has transaction.name: new_name -- the transaction name at the time the transaction was ended.