Fix ValueError('Incorrect label names')

[sven-rosenzweig]

When request errors occur (e.g. ConnectTimeout),
the path label was not added to the Prometheus metric, causing a ValueError.

[github-actions]

Name                                                                      Stmts   Miss  Cover
---------------------------------------------------------------------------------------------
networking_nsxv3/api/rpc.py                                                 250    121    52%
networking_nsxv3/common/config.py                                            16      0   100%
networking_nsxv3/common/constants.py                                         23      0   100%
networking_nsxv3/common/locking.py                                           35     11    69%
networking_nsxv3/common/synchronization.py                                  378     73    81%
networking_nsxv3/db/db.py                                                   119     19    84%
networking_nsxv3/extensions/nsxtoperations.py                               110     29    74%
networking_nsxv3/extensions/nsxtpolicy.py                                    84      7    92%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/agent.py                   174     60    66%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/cli.py                     302    199    34%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/client_nsx.py              211     51    76%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/constants_nsx.py             6      0   100%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/extensions/firewall.py      27      0   100%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/provider.py                171     10    94%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/provider_nsx_policy.py     778    113    85%
networking_nsxv3/plugins/ml2/drivers/nsxv3/agent/realization.py             238     59    75%
networking_nsxv3/plugins/ml2/drivers/nsxv3/driver.py                        143     74    48%
networking_nsxv3/prometheus/exporter.py                                      21      5    76%
networking_nsxv3/services/logapi/drivers/nsxv3/driver.py                     41      1    98%
networking_nsxv3/services/qos/drivers/nsxv3/qos.py                           34      4    88%
networking_nsxv3/services/trunk/drivers/nsxv3/trunk.py                       71      3    96%
---------------------------------------------------------------------------------------------
TOTAL                                                                      3232    839    74%