Adds ifAlias label to all exported metrics.

[nkinkade]

It is handy to have an ifAlias label on all switch metrics so that we can easily query for a particular machine or the "uplink" without having to fuss with interface names (e.g., xe-0/0/12, xe-0/0/45, ge-0/0/47, etc.). ifAlias is currently added to all metrics collected with snmp_exporter. This PR adds an ifAlias label to all metrics exported by DISCOv2.

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This change is 

[stephen-soltesz]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nkinkade and @stephen-soltesz)

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metrics/metrics.go, line 77 at r1 (raw file):

			oidMap, err := getOidsString(client, []string{ifDescrOid})
			rtx.Must(err, "Failed to determine the machine interface ifDescr")
			ifaces["machine"]["ifAlias"] = machine

just to check, sometimes machine means "mlab1" and sometimes it means the fqdn. in this case, the label is just the short form?

[nkinkade]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @stephen-soltesz)

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metrics/metrics.go, line 77 at r1 (raw file):

Previously, stephen-soltesz (Stephen Soltesz) wrote…

just to check, sometimes machine means "mlab1" and sometimes it means the fqdn. in this case, the label is just the short form?

That's right. In this case, machine refers only to mlab[1-4]: https://github.com/m-lab/disco/blob/master/metrics/metrics.go#L259. But you're right that maybe I've chosen a bad name here, since it is possibly ambiguous, especially if I've used "machine" to mean the FQDN in other places.