Add exporter subcommand

[rossf7]

Towards #25

This moves the exporter code from grid-intensity-exporter and adds the WattTime provider.

Also the grid_intensity_carbon_actual metric is renamed to grid_intensity_carbon_average and the units are added.

go run . exporter -r ES
Using provider "ember-climate.org" with region "ES"
Metrics available at :8000/metrics

curl -s http://localhost:8000/metrics | grep grid
# HELP grid_intensity_carbon_average Average carbon intensity for the electricity grid in this region.
# TYPE grid_intensity_carbon_average gauge
grid_intensity_carbon_average{provider="ember-climate.org",region="ES",units="gCO2 per kWh"} 193.737

For the WattTime provider both the marginal and relative metrics are output if they are present.

go run . exporter -p watttime.org -r CAISO_NORTH
Using provider "watttime.org" with region "CAISO_NORTH"
Metrics available at :8000/metrics

curl -s http://localhost:8000/metrics | grep grid
# HELP grid_intensity_carbon_marginal Marginal carbon intensity for the electricity grid in this region.
# TYPE grid_intensity_carbon_marginal gauge
grid_intensity_carbon_marginal{provider="watttime.org",region="CAISO_NORTH",units="lb CO2 per MWh"} 911
# HELP grid_intensity_carbon_relative Relative carbon intensity for the electricity grid in this region.
# TYPE grid_intensity_carbon_relative gauge
grid_intensity_carbon_relative{provider="watttime.org",region="CAISO_NORTH",units="percent"} 71

Otherwise just the relative intensity is output.

go run . exporter -p watttime.org -r ES
Using provider "watttime.org" with region "ES"
Metrics available at :8000/metrics

curl -s http://localhost:8000/metrics | grep grid
# HELP grid_intensity_carbon_relative Relative carbon intensity for the electricity grid in this region.
# TYPE grid_intensity_carbon_relative gauge
grid_intensity_carbon_relative{provider="watttime.org",region="ES",units="percent"} 20

[rossf7]

@mrchrisadams This is ready for review.

This is already pretty big so the next PR will move the docker, k8s and nomad code along with their integration tests.

[mrchrisadams]

I'm really happy with this PR Ross, thanks! Please merge in at your convenience 👍