Add a basic monitoring endpoint to the sshd command.
* Listen on localhost port 9122 by default.
* Integrate build/version info.
* Update example config.

https://gitlab.com/gitlab-org/gitlab-shell/-/issues/121

Signed-off-by: Ben Kochie <superq@gmail.com>

The config.yml.example didn't include a field I was expecting to be
there, which lead me to believe the field didn't exist. This change adds
the `secret` YAML field, and describes how it interacts with the
secrets_file.

From
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4498#note_397401883,
if you specify a relative path such as:

```
external_url 'http://gitlab.example.com/gitlab'
```

gitlab-shell doesn't have a way to pass the `/gitlab` to the host. For example, let's say we have:

```
gitlab_url: "http+unix://%2Fvar%2Fopt%2Fgitlab%2Fgitlab-workhorse%2Fsocket"
```

If we have `/gitlab` as the relative path, how do we specify what is the
UNIX socket path and what is the relative path? If we specify:

```
gitlab_url: "http+unix:///var/opt/gitlab/gitlab-workhorse.socket/gitlab
```

This is ambiguous. Is the socket in
`/var/opt/gitlab/gitlab-workhorse.socket/gitlab` or in
`/var/opt/gitlab/gitlab-workhorse.socket`?

To fix this, this merge request adds an optional
`gitlab_relative_url_root` config parameter:

```
gitlab_url: "http+unix://%2Fvar%2Fopt%2Fgitlab%2Fgitlab-workhorse%2Fsocket"
gitlab_relative_url_root: /gitlab
```

This is only used with UNIX domain sockets to disambiguate the socket
and base URL path. If `gitlab_url` uses `http://` or `https://`, then
`gitlab_relative_url_root` is ignored.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/476

The unicorn replacement 'puma' uses a unix socket in the example config [1] instead of a tcp port.
Using the non-existing tcp port results in "Internal API unreachable" on
git operations.

[1] https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/puma.rb.example#L34

It now lives within gitaly

In order to uncomment it in the Makefile of GDK