Add node and region labels to exporter

[rossf7]

Towards #59

This adds --node and --region flags to the exporter command. The GRID_INTENSITY_NODE and GRID_INTENSITY_REGION env vars can also be used to set this.

go run . exporter -h
A prometheus exporter for getting the carbon intensity data for
electricity grids.

This can be used to make your software carbon aware so it runs at times when
the grid is greener or at locations where carbon intensity is lower.

	grid-intensity exporter --provider Ember --location IE --region eu-west-1 --node worker-1
	grid-intensity exporter -p Ember -l BOL

Usage:
  grid-intensity exporter [flags]

Flags:
  -h, --help              help for exporter
  -l, --location string   Location code for provider
  -n, --node string       Node where the exporter is running
  -p, --provider string   Provider of carbon intensity data (default "Ember")
  -r, --region string     Region where the exporter is running

The labels get added to the metrics like here.

curl -s http://localhost:8000/metrics | grep grid
# HELP grid_intensity_carbon_average Average carbon intensity for the electricity grid in this location.
# TYPE grid_intensity_carbon_average gauge
grid_intensity_carbon_average{location="IE",node="kind-kind",provider="Ember",region="eu-west-1",units="gCO2e per kWh"} 361.274

[mrchrisadams]

hi @rossf7 - this should be unblocked now, thanks for being patient with me 😅

[rossf7]

Setting the node name to make it predictable in the test.

[rossf7]

The node name is set automatically.

[rossf7]

For k8s the region has to be set by the user. As there is no easy way to get the node label even if its set which is a pain.

[rossf7]

For nomad we can template both the node and region which is kinda nice :)

[rossf7]

@mrchrisadams Thanks this one is ready for review as well now.

@fershad Could you add these extra params to the docs changes?

[mrchrisadams]

Hi Ross,

thanks for adding all the explanatory comments. I'm happy for this to be merged in.