Add nb-connection-status & sb-connection-status metrics for ovn-northd

[girishmg]

We would like to know the status of OVN DB connection from ovn-northd to both NB and SB. Once we have the metric, then we can set alert on the same.

@trozet @tssurya PTAL

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[coveralls]

coverage: 52.293% (-0.05%) from 52.346% when pulling 4e6ab6414d57a0e6e8511ef8f77f570dc922c576 on girishmg:ovn-northd-metrics into ebdc80f754377601b6865f86fb2c9c9e90e84dee on ovn-org:master.