feat(baremetal): read the traffic graphs, without knowing a MAC address

[Denis-hamon]

Stacked on #253. Only the last commit belongs to this pull request.

The CLI reached none of the traffic data

Three routes carry it, and two are dead ends:

route	state
/dedicated/server/{sn}/mrtg	deprecated 2017-10-23, deletion announced 2018-04-23
/dedicated/server/{sn}/vrack/{vrack}/mrtg	deprecated 2017-10-23, marked LEGACY
/dedicated/server/{sn}/networkInterfaceController/{mac}/mrtg	live

Both deprecated ones name /networkInterfaceController as their replacement,
so only that one is wired here.

The route wants a MAC address, and nothing could tell you one

No command lists the controllers of a server. vni list shows the virtual
interfaces, and their own MAC list appears only under -o json. Asking an
operator for a MAC they have no way to look up would be a command nobody can
run, so traffic resolves the server to its controllers and reads every one.

ovhcloud baremetal traffic <server> [--period daily]
                                    [--type traffic:download,traffic:upload]
                                    [--nic <mac>]

Both directions are read by default: how much is this server doing is a
question about both, and answering one of them silently answers half.

Periods and types are checked against the API enums before the call, so a typo
comes back with the list of what would have worked rather than as a 400.

Numbers appear twice, on purpose

Measured against a real ADVANCE-1, on the day it was reinstalled:

nic                type              peak         average      latest
9c:6b:00:a9:c5:10  traffic:download  33.98 Mbps   233.84 kbps  46.92 kbps
9c:6b:00:a9:c5:10  traffic:upload    116.23 kbps  1.69 kbps    2.32 kbps
9c:6b:00:a9:ca:25  traffic:download  944.68 bps   774.36 bps   739.16 bps
9c:6b:00:a9:ca:25  traffic:upload    15.73 bps    0.17 bps     0 bps

That peak is the operating system image being downloaded during the reinstall.

The raw figures behind those cells are 33978092.769 and an average of
0.16518435754189942 — true, and useless to read. So the table shows a scaled
rate, while -o json and -o peak keep the raw values:

$ ovhcloud baremetal traffic <server> -o peak
33978092.769

-o json also carries the whole series. A day is 173 samples, which no
table can show and every script wants; dropping it would make the CLI's output
poorer than the API it wraps.

Every unit this API returns is a rate — bps, pps and eps, all three seen
on a real server — so one scale serves all three: 33.98 Mbps, 2.87 kpps,
0 eps.

Verification

go build, go vet, make wasm, go test ./..., docgen — all green.

10 tests. 7 sabotages, 7 red, each on the test meant to catch it: the scale
removed, the series dropped, latest replaced by the peak, an empty series
reported as zero, only the first controller read, and each of the two enum
checks disabled.

One of those sabotages first reported no test falls — it disabled the error
branch of controllersOf rather than shortening its result, so it never
restricted anything. Re-applied where it belonged, the test fell. The zero came
from the instrument, not from the code.

Left out deliberately

GET/PUT /dedicated/server/{sn}/burst is live and uncovered, but it answers
404 "The requested object (burst) does not exist" on 8 of the 8 servers
checked. The read path could be shipped and would report that answer, but the
path that returns a burst configuration cannot be exercised on this account, so
there is no command for it here.