Document missing azure and gce meta labels (#682)
Document missing azure and gce meta labels (#682)

* Document __meta_azure_machine_name
* Document __meta_gce_metadata_*
* Correct __meta_gce_tags_* (remove __meta_gce_instance_tags_*)
* Sort long lists alphabetically
* Link to relabeling section from all meta label lists.
prometheus/docs,Cepave/openfalcon.io,open-falcon/open-falcon.github.io,chosenken/docs,chosenken/docs,open-falcon/open-falcon.github.io,du2016/prometheus-docs,ddfreyne/prometheus-docs,prometheus/docs,chosenken/docs,ddfreyne/prometheus-docs,prometheus/docs,ddfreyne/prometheus-docs,open-falcon/open-falcon.github.io,Cepave/openfalcon.io,du2016/prometheus-docs,chosenken/docs,Cepave/openfalcon.io,du2016/prometheus-docs,ddfreyne/prometheus-docs