Remove transport.Default

[axw]

Remove transport.Default, and construct a new transport for each new tracer. This will enable us to pass the tracer's associated service name and version into its transport, for appending to the User-Agent header.

Closes #1194

[apmmachine]

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• Start Time: 2022-01-14T07:37:21.255+0000

• Duration: 32 min 54 sec

• Commit: 1df6b92

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[marclop]

Thank you for adding the missing notes to the change log, I've been meaning to do so, but hadn't gotten to it.

[marclop]

I verified that transport.Default doesn't resolve to and that my Go program does not compile 😄

package main

import (
	"go.elastic.co/apm/v2/transport"
)

func main() {
	transport.Default
}

$ go run a/main.go
# command-line-arguments
a/main.go:25:2: undefined: transport.Default