relay: Add connection stats to MetricsPort

This adds the number of created and opened connections to the
MetricsPort for a relay for each connection type and direction.

Output looks like:

  # HELP tor_relay_connections Connections metrics of this relay
  # TYPE tor_relay_connections counter
  tor_relay_connections{type="OR listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="OR listener",direction="received",state="created"} 0
  tor_relay_connections{type="OR listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="OR listener",direction="received",state="opened"} 0
  tor_relay_connections{type="OR",direction="initiated",state="created"} 5
  tor_relay_connections{type="OR",direction="received",state="created"} 0
  tor_relay_connections{type="OR",direction="initiated",state="opened"} 5
  tor_relay_connections{type="OR",direction="received",state="opened"} 0
  tor_relay_connections{type="Exit",direction="initiated",state="created"} 0
  tor_relay_connections{type="Exit",direction="received",state="created"} 0
  tor_relay_connections{type="Exit",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Exit",direction="received",state="opened"} 0
  tor_relay_connections{type="Socks listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Socks listener",direction="received",state="created"} 0
  tor_relay_connections{type="Socks listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Socks listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Socks",direction="initiated",state="created"} 0
  tor_relay_connections{type="Socks",direction="received",state="created"} 0
  tor_relay_connections{type="Socks",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Socks",direction="received",state="opened"} 0
  tor_relay_connections{type="Directory listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Directory listener",direction="received",state="created"} 0
  tor_relay_connections{type="Directory listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Directory listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Directory",direction="initiated",state="created"} 0
  tor_relay_connections{type="Directory",direction="received",state="created"} 0
  tor_relay_connections{type="Directory",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Directory",direction="received",state="opened"} 0
  tor_relay_connections{type="Control listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Control listener",direction="received",state="created"} 0
  tor_relay_connections{type="Control listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Control listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Control",direction="initiated",state="created"} 0
  tor_relay_connections{type="Control",direction="received",state="created"} 0
  tor_relay_connections{type="Control",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Control",direction="received",state="opened"} 0
  tor_relay_connections{type="Transparent pf/netfilter listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Transparent pf/netfilter listener",direction="received",state="created"} 0
  tor_relay_connections{type="Transparent pf/netfilter listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Transparent pf/netfilter listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Transparent natd listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Transparent natd listener",direction="received",state="created"} 0
  tor_relay_connections{type="Transparent natd listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Transparent natd listener",direction="received",state="opened"} 0
  tor_relay_connections{type="DNS listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="DNS listener",direction="received",state="created"} 0
  tor_relay_connections{type="DNS listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="DNS listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Extended OR",direction="initiated",state="created"} 0
  tor_relay_connections{type="Extended OR",direction="received",state="created"} 0
  tor_relay_connections{type="Extended OR",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Extended OR",direction="received",state="opened"} 0
  tor_relay_connections{type="Extended OR listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Extended OR listener",direction="received",state="created"} 0
  tor_relay_connections{type="Extended OR listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Extended OR listener",direction="received",state="opened"} 0
  tor_relay_connections{type="HTTP tunnel listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="HTTP tunnel listener",direction="received",state="created"} 0
  tor_relay_connections{type="HTTP tunnel listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="HTTP tunnel listener",direction="received",state="opened"} 0
  tor_relay_connections{type="Metrics listener",direction="initiated",state="created"} 0
  tor_relay_connections{type="Metrics listener",direction="received",state="created"} 1
  tor_relay_connections{type="Metrics listener",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Metrics listener",direction="received",state="opened"} 1
  tor_relay_connections{type="Metrics",direction="initiated",state="created"} 0
  tor_relay_connections{type="Metrics",direction="received",state="created"} 0
  tor_relay_connections{type="Metrics",direction="initiated",state="opened"} 0
  tor_relay_connections{type="Metrics",direction="received",state="opened"} 0

Related to #40194

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket40194

@@ -0,0 +1,3 @@
+  o Minor feature (relay, metrics):
+    - Add counters to the MetricsPort how many connections, per type, are
+      currently opened and how many were created. Part of ticket 40194.