Pushgateway exposition: properly handle empty label values in grouping key

[beorn7]

See prometheus/pushgateway#346 for a documentation update for the PGW about empty label values in the grouping key. (Note: It's really just documenting the current state of the PGW. No change of behavior of the PGW involved.)

The code in exposition.py currently accepts empty label values for the grouping key. However, it then attempts to push in an invalid way, see prometheus/pushgateway#344 for details (the issue demonstrates it with client_golang, but presumably client_python would show exactly the same behavior).

While the use case for empty label values in the grouping key is somewhat esoteric, a case can be made for it. Thus, we should probably support it in the way now documented. Alternative is to disallow it in exposition.py. Just the current state of pretending it works but then fail to push is clearly a bug.