Add Connection Status Metric

[ltcarbonell]

This adds a new telemetry metric that tracks the connection status of the follower nodes.

Example output:

$ curl -sk -H "X-Vault-Request: true" -H "X-Vault-Token: token" http://127.0.0.1:8200/v1/sys/metrics | jq | grep -A 3 -B 1 'connection_status'
    {
      "Name": "vault.raft_storage.follower.connection_status",
      "Value": 1,
      "Labels": {
        "peer_id": "vault-1"
--
    {
      "Name": "vault.raft_storage.follower.connection_status",
      "Value": 1,
      "Labels": {
        "peer_id": "vault-2"
$ docker stop vault-1
vault-1
$ curl -sk -H "X-Vault-Request: true" -H "X-Vault-Token: token" http://127.0.0.1:8200/v1/sys/metrics | jq | grep -A 3 -B 1 'connection_status'
    {
      "Name": "vault.raft_storage.follower.connection_status",
      "Value": 0,
      "Labels": {
        "peer_id": "vault-1"
--
    {
      "Name": "vault.raft_storage.follower.connection_status",
      "Value": 1,
      "Labels": {
        "peer_id": "vault-2"

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CI Results: All Go tests succeeded! :white_check_mark:

[github-actions[bot]]

Build Results: All builds succeeded! :white_check_mark:

[digivava]

Changed the milestone on this to 1.16.1 since it's still a draft and today is code freeze