Create Stat Panel, Update Gauge Panel

[trotttrotttrott]

Stat and gauge are modeled very similarly so I put them together in this PR.

I've tested them with both Grafana 7.0.5 (latest release) and 6.7.4 (latest release of major version 6). These can be selected by passing the pluginVersion parameter to either panel. Grafana version support was the biggest challenge this PR attempts to address. While Grafana does a good job of adapting older JSON models to current versions, we'd like to keep how we model them Grafonnet current so we can support new features.

Closes #237.

Closes #221. This PR was cherry picked to get started on the gauge panel updates.

[fty4]

Is there any option to specify width / length of the new gauge panel?

In the old gauge there was a option to set span and height.

[trotttrotttrott]

@fty4 it's intended that gridPos is used.

grafana.gaugePanel.new(
  'Example'
) {
  gridPos: { h: 6, w: 8, x: 0, y: 0 }
}

This field has been used for layout since version 5. However, Grafana will convert the height and span fields when dashboards are loaded. I think I had it in mind that we'd be retiring span and height, but perhaps that's not the right move. I'll create an issue to discuss.

In the meantime, you can add them like in the gridPos example:

grafana.gaugePanel.new(
  'Example'
) {
  height: ...,
  span: ...,
}