Address issue argoproj#1403 by adding a Graphite
metrics provider.

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Per code review feedback...

> It's a bad idea to use the golang HTTP default client because of possibility of hung connections, since it does not timeout

As outlined here:
https://medium.com/@nate510/don-t-use-go-s-default-http-client-4804cb19f779

The 10s timeout matches the default of the webmetric provider's HTTP client.

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Previously, `graphite.API#Query` returned the value of
the last item in the `datapoints` array returned by
the Graphite API. However, this somewhat diverged from
the Prometheus metricprovider implementation and fails
to consider a use case for obtaining the first or an
arbitrary element from the datapoints array, as
discussed here:

argoproj#1406 (comment)

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Mike Ball <mikedball@gmail.com>

…cs-provider

Signed-off-by: Mike Ball <mikedball@gmail.com>

Signed-off-by: Jesse Suen <jessesuen@gmail.com>