WIP : Expose active/inactive connection to service backend as prometheus metrics

[murali-reddy]

partial fix for #5

[murali-reddy]

metrics exposed as http://nodeip:8080/metrics

# HELP kube_router_service_backend_active_connections Active conntection to backend of service
# TYPE kube_router_service_backend_active_connections gauge
kube_router_service_backend_active_connections{backend="100.96.1.2",namespace="kube-system",service_name="kube-dns"} 0
kube_router_service_backend_active_connections{backend="100.96.1.3",namespace="default",service_name="redis-slave"} 0
kube_router_service_backend_active_connections{backend="100.96.1.4",namespace="default",service_name="frontend"} 0
kube_router_service_backend_active_connections{backend="100.96.1.5",namespace="default",service_name="frontend"} 0
kube_router_service_backend_active_connections{backend="100.96.2.3",namespace="kube-system",service_name="kube-dns"} 0
kube_router_service_backend_active_connections{backend="100.96.2.4",namespace="default",service_name="redis-master"} 1
kube_router_service_backend_active_connections{backend="100.96.2.5",namespace="default",service_name="redis-slave"} 0
kube_router_service_backend_active_connections{backend="100.96.2.6",namespace="default",service_name="frontend"} 0
kube_router_service_backend_active_connections{backend="172.20.51.157",namespace="default",service_name="kubernetes"} 1
# HELP kube_router_service_backend_inactive_connections Active conntection to backend of service
# TYPE kube_router_service_backend_inactive_connections gauge
kube_router_service_backend_inactive_connections{backend="100.96.1.2",namespace="kube-system",service_name="kube-dns"} 0
kube_router_service_backend_inactive_connections{backend="100.96.1.3",namespace="default",service_name="redis-slave"} 0
kube_router_service_backend_inactive_connections{backend="100.96.1.4",namespace="default",service_name="frontend"} 0
kube_router_service_backend_inactive_connections{backend="100.96.1.5",namespace="default",service_name="frontend"} 0
kube_router_service_backend_inactive_connections{backend="100.96.2.3",namespace="kube-system",service_name="kube-dns"} 0
kube_router_service_backend_inactive_connections{backend="100.96.2.4",namespace="default",service_name="redis-master"} 2
kube_router_service_backend_inactive_connections{backend="100.96.2.5",namespace="default",service_name="redis-slave"} 0
kube_router_service_backend_inactive_connections{backend="100.96.2.6",namespace="default",service_name="frontend"} 0
kube_router_service_backend_inactive_connections{backend="172.20.51.157",namespace="default",service_name="kubernetes"} 0