proxy: export per-router metrics

[olix0r]

each place we use a router, we should be able to track metrics including:

• router_insert_total -- counter tracking the total number of insertions into the router
• router_expiry_total -- counter tracking the total number of expirations from the router
• router_rejects_total -- counter tracking the total number of insert attempts that were rejected due to capacity... Alternatively, this may be end up being a router_oneshot_total (tbd).

Followup from linkerd/linkerd2-proxy#247

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.