Update tests and doc comments to use SERVER_URL

[axw]

The Go Agent will continue to support SERVER_URLS,
but we will de-epmphasise its use in favour of using
load balancer proxies with agents generally. We never
actually documented SERVER_URLS in the user docs,
so nothing to do there.

Closes #850

[apmmachine]

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• Build Cause: Pull request #858 opened

• Start Time: 2020-12-10T02:24:30.127+0000

• Duration: 25 min 27 sec

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Total	7838

[simitt]

Just a minor thing, but since ELASTIC_APM_SERVER_URLS is deprecated, should we also change the order of how the ENV vars are parsed, so that ELASTIC_APM_SERVER_URL is always used if provided.

apm-agent-go/transport/http.go

Lines 528 to 538 in 8b1a0fe

	// initServerURLs parses ELASTIC_APM_SERVER_URLS if specified,
	// otherwise parses ELASTIC_APM_SERVER_URL if specified. If
	// neither are specified, then the default localhost URL is
	// returned.
	func initServerURLs() ([]*url.URL, error) {
	key := envServerURLs
	value := os.Getenv(key)
	if value == "" {
	key = envServerURL
	value = os.Getenv(key)
	}

.

[axw]

I did think about doing that, and decided at the time to make it a purely non-functional change. I don't think it's likely to matter either way, but I'd like to avoid a breaking change unless necessary.

[axw]

I assume since you approved that you're good either way, so I'll merge. If you disagree with my decision, let me know and we can come back to it.

[simitt]

yes, I am fine either way - thanks for the explanation.