Add inbound policy index metrics

[adleong]

We add index size gauges for each of the resource indexes in the inbound policy index.

# HELP inbound_index_meshtls_authentication_index_size The number of MeshTLS authentications in index
# TYPE inbound_index_meshtls_authentication_index_size gauge
inbound_index_meshtls_authentication_index_size{namespace="linkerd-viz"} 1
# HELP inbound_index_network_autnetication_index_size The number of Network authentications in index
# TYPE inbound_index_network_autnetication_index_size gauge
inbound_index_network_autnetication_index_size{namespace="linkerd-viz"} 2
# HELP inbound_index_pod_index_size The number of pods in index
# TYPE inbound_index_pod_index_size gauge
inbound_index_pod_index_size{namespace="linkerd"} 3
inbound_index_pod_index_size{namespace="emojivoto"} 4
inbound_index_pod_index_size{namespace="linkerd-viz"} 5
# HELP inbound_index_external_workload_index_size The number of external workloads in index
# TYPE inbound_index_external_workload_index_size gauge
inbound_index_external_workload_index_size{namespace="linkerd"} 0
inbound_index_external_workload_index_size{namespace="emojivoto"} 0
inbound_index_external_workload_index_size{namespace="linkerd-viz"} 0
# HELP inbound_index_server_index_size The number of servers in index
# TYPE inbound_index_server_index_size gauge
inbound_index_server_index_size{namespace="linkerd"} 0
inbound_index_server_index_size{namespace="emojivoto"} 1
inbound_index_server_index_size{namespace="linkerd-viz"} 4
# HELP inbound_index_server_authorization_index_size The number of server authorizations in index
# TYPE inbound_index_server_authorization_index_size gauge
inbound_index_server_authorization_index_size{namespace="linkerd"} 0
inbound_index_server_authorization_index_size{namespace="emojivoto"} 0
inbound_index_server_authorization_index_size{namespace="linkerd-viz"} 0
# HELP inbound_index_authorization_policy_index_size The number of authorization policies in index
# TYPE inbound_index_authorization_policy_index_size gauge
inbound_index_authorization_policy_index_size{namespace="linkerd"} 0
inbound_index_authorization_policy_index_size{namespace="emojivoto"} 0
inbound_index_authorization_policy_index_size{namespace="linkerd-viz"} 4
# HELP inbound_index_http_route_index_size The number of HTTP routes in index
# TYPE inbound_index_http_route_index_size gauge
inbound_index_http_route_index_size{namespace="linkerd"} 0
inbound_index_http_route_index_size{namespace="emojivoto"} 600
inbound_index_http_route_index_size{namespace="linkerd-viz"} 0