all: use Node and Resource to derive default labels

[odeke-em]

I left a TODO in

prometheus-go-metrics-exporter/prometheus.go

Line 174 in d0aca57

// TODO: (@odeke-em) use node, resource and metrics e.g. perhaps to derive default labels

to derive default labels. The OpenCensus-Go stats Prometheus exporter doesn't yet derive default labels from resource but now that this metrics exporter is in use, it would be useful to derive them from forwarded Metrics.