Add outbound policy index metrics

[adleong]

Adds index metrics to the outbound policy index.

# HELP outbound_policy_index_size Gauge of the number of resources in the index.
# TYPE outbound_policy_index_size gauge
outbound_policy_index_size{namespace="linkerd",kind="Service"} 8
outbound_policy_index_size{namespace="emojivoto",kind="Service"} 2
outbound_policy_index_size{namespace="default",kind="Service"} 2
outbound_policy_index_size{namespace="emojivoto",kind="HTTPRoute"} 0
outbound_policy_index_size{namespace="linkerd-viz",kind="Service"} 5
outbound_policy_index_size{namespace="kube-system",kind="Service"} 3
outbound_policy_index_size{namespace="cert-manager",kind="Service"} 2
# HELP outbound_policy_index_applies Count of applies to the index.
# TYPE outbound_policy_index_applies counter
outbound_policy_index_applies_total{namespace="linkerd",kind="Service"} 8
outbound_policy_index_applies_total{namespace="emojivoto",kind="HTTPRoute"} 600
outbound_policy_index_applies_total{namespace="linkerd-viz",kind="Service"} 5
outbound_policy_index_applies_total{namespace="kube-system",kind="Service"} 3
outbound_policy_index_applies_total{namespace="emojivoto",kind="Service"} 2
outbound_policy_index_applies_total{namespace="cert-manager",kind="Service"} 2
outbound_policy_index_applies_total{namespace="default",kind="Service"} 2
# HELP outbound_policy_index_deletes Counte of deletes to the index.
# TYPE outbound_policy_index_deletes counter
# HELP outbound_policy_index_resets Count of resets to the index.
# TYPE outbound_policy_index_resets counter

[adleong]

replaced by #12429