Add link capacity/utilization metrics

[dmke]

This adds up/downlink capacity and utilization metrics. They are optional and can be enabled with --extra-metrics=link.

I'm currently not 100% clear on a few points, so I'll leave this PR as draft for now. I'm going to run this for a few days, and maybe update the PR accordingly:

Wifi-bridges only?

I don't know to which interface the link fields belong. It might be, that these metrics are only available for WiFi bridges (such as Nano- or Gigabeam devices).

The numbers retrieved from the API correspond at least with this observation.

If this turn out to be true, I might rename the extra metric to something like wireless-link, or wifi-bridge, so similar.

Mean calculation

Currently, the response returned from /devices/{id}/statistics contains (at minimum) 1 full hour of link data, and it seems they are reported in 15s intervals. The capacity values look highly volatile, as do the utilization ratios (they represent a function of network traffic / capacity).

To limit the amount of data to be computed, I've opted to only take the last 10 minutes into account. To combat the volatility (but not loose anomalies, like a sudden drop in capacity) I'm calculating a weighted mean over the data, where the weight for a data point decreases as its age increases.

I've implemented a linear interpolation for the weight between "now" and "10 min ago", but I might switch to a exponential interpolation and/or adjust the window size.

Comparison curves

weight  ↑                             weight  ↑
     1 -+               · ·                1 -+              · ·
        |             ·                       |              ·
        |           ·                         |             .
        |         ·                           |            . 
     0 -+ · · · ·                          0 -+ . . . . · `
        |-------+-------+-→ time              |------+-------+-→ time
             t-10min    now                       t-10min    now